Mg Of Testosterone Propionate Research Articles

Testosterone-induced enhancement of male medial preoptic tissue transplant volume in female recipients: a possible neuronotrophic action

To characterize further the action of gonadal hormones on the development of the brain, this study was designed to test the ability of testosterone treatment to modify the volume of male brain tissue transplants in female recipients. One-day-old females, in addition to having either medial preoptic area (MPOA) or caudate nucleus (CN) tissue from neonatal males bilaterally implanted into their own MPOA, also received a subcutaneous injection of either 200 μg testosterone propionate (TP) or oil concurrently, and on the following 4 days. All recipients were sacrificed at 30 days of age postnatally. An analysis of male transplant volumes indicated that MPOA transplants in oil-treated recipients were substantially reduced in size (0.06 ± 0.01 mm 3) compared with the initial transplant volume of 0.19 mm 3. However, MPOA transplants in recipients treated with TP showed a 79% increase above the initial transplant volume (to 0.34 ± 0.05 mm 3). The resulting 5- to 6-fold difference in MPOA transplant volume between oil and TP treated animals was highly significant. In sharp contrast, the same TP treatment to recipients receiving male CN transplants resulted in no enhancement of transplant volume. Therefore, transplants involving a brain area known to concentrate 3H-labeled testosterone neonatally (i.e. the MPOA) responded to TP treatment with an enhancement of volume which was not observed for transplants consisting of brain tissue not known to concentrate 3H-labeled testosterone (i.e. the CN). The above results suggest that testosterone is a ‘neuronotrophic” agent during development that acts specifically on cells within steroid-sensitive brain areas, perhaps to prevent neuronal death within these areas.

Testosterone and its metabolites in male cynomolgus monkeys ( Macaca fascicularis): Behavior and biochemistry

To extend our previous study [22] on the behavioral effects of testosterone propionate (TP) and dihydrotestosterone propionate (DHTP) to a dose-range producing supra-physiological plasma androgen levels, 4 castrated cynomolgus monkeys were tested with the same 4 females during successive 4-week treatment periods while receiving 800 μg, 1.6 mg, 3.2 mg, 6.4 mg and 12.8 mg of TP or DHTP SC/day in counterbalanced order (16 pairs, 828 1-hr tests). Both androgens increased male sexual activity, but DHTP was less effective than TP in increasing the numbers of ejaculations per test and failed to restore ejeculations to intact levels. Giving androgen-treated males single injections of 50 μg and 100 μg estradiol benzoate (EB) was without any additional effect on behavior (16 pairs, 256 test). To examine hormonal effects in the brain, castrated males were given either 3H-T or 3H-DHT, and tissues were examined by high performance liquid chromatography (hplc). After 3H-T, 3H-E 2 and unchanged 3H-T were the major forms of radioactivity in nuclei from hypothalamus, preoptic area and amygdala. After 3H-DHT, unchanged 3H-DHT predominated. The lower behavioral effectiveness of DHT could not be ascribed to its failure to enter the brain. The data suggested a role for unchanged T in the regulation of ejaculatory behavior in a male primate.

Neonatal androgenization in ground squirrels: influence on sex differences in body mass and luteinizing hormone levels.

Female squirrels were injected at birth with 50 or 1000 micrograms testosterone propionate (TP); control males and females were treated with oil vehicle. Squirrels were gonadectomized at 47 days of age. Body mass was recorded weekly and plasma luteinizing hormone (LH) was determined once monthly over the next year. Marked annual cycles in body mass were manifested by 30 out of 31 squirrels. Peak body mass and peak-to-trough differences were greater for control male and TP-female squirrels than for control female squirrels. Trough body weights did not differ among the groups. Luteinizing hormone was detectable in all male and most androgenized females but not in any control female squirrels during the first 4 mo after gonadectomy. Peak LH values were significantly greater for control male than for control female squirrels and were not influenced by neonatal androgenization in females. Testosterone propionate treatment also did not affect sex differences in timing of LH peaks or the total number of months in which LH was detectable. We conclude that testicular hormones secreted during the early postnatal period induce sex differences in the circannual pattern of weight change and some aspects of LH secretion. Complete masculinization, however, either requires more extensive action of gonadal hormones, perhaps both pre- and postnatally, or occurs through some androgen-independent mechanism.

Prenatal androgen exposure, preoptic area and reproductive functions in the female rat

Pregnant rats were injected with 4 mg of testosterone propionate (TP) from days 15 to 22 of pregnancy or a single dose of 4 mg TP on day 17 or 21 of pregnancy. Female offspring treated with TP or the vehicle prenatally were sacrificed at 90 days of age. The volume of the cluster of intensely stained neurons in the medial preoptic area, the "sexually dimorphic nucleus of the preoptic area" (SDN-POA), was found to be affected by prenatal exposure to TP. The effect of TP on the development of the SDN-POA was most remarkable in the animals treated with a single dose of TP on day 17 of pregnancy, whereas TP injection on day 21 of pregnancy had no remarkable effect. These results suggest that continuous prenatal exposure to TP is not essential for stimulation of the development of the SDN-POA, suggesting the presence of a critical period during which the SDN-POA is most sensitive to the androgen. Although 4 out of 11 females exposed to TP from days 15 to 22 were sterile, the ovarian function of most of the TP-exposed offspring was normal. Furthermore, prenatally TP-exposed females failed to show behavioral masculinization and defeminization. All these females displayed high lordotic response levels and only rarely showed mounting behavior. These results suggest that the absolute volume of the SDN-POA does not seem to be directly correlated with the regulation of the cyclic release of gonadotropins and/or expression of sexual behavior.

Social behaviour and testicular activity of juvenile rats.

The juvenile stage of ontogeny is often characterized as a time of inactivity and quiescence for the immature reproductive system. The principle social behaviour by juveniles of many mammalian species is a rough-and-tumble activity known as play-fighting. An experiment is reported in which play-fighting by male rats was observed after various manipulations of gonadal steroids. Rats were housed in groups and castrated either on day 1 or day 10, times which are respectively during and after the sensitive period for androgen-induced organization of neural tissues in rats. Animals were injected with either 40 micrograms testosterone propionate or vehicle between 21 and 45 days of age, and play-fighting with unoperated, unfamiliar rats was examined. Castration on days 1 and 10 suppressed play-fighting. The behaviours remained suppressed in rats castrated on day 1 and injected with testosterone propionate, but testosterone propionate restored play-fighting to near-normal levels until 35 days of age in rats castrated on day 10. Moreover, gonadally intact juvenile males exposed to the androgen antagonist flutamide play-fought less than intact control males, although their social activity increased appreciably after day 35. It is concluded that the juvenile is not experiencing endocrine quiescence. Behavioural and physiological data suggest a reproductive system which is active, although differently from that in the adult. Hypersensitivity to testosterone and surges of gonadotrophin-releasing factors and LH at 35 days of age may be the events responsible for changes in play-fighting resulting from manipulations of gonadal steroids in juvenile rats.

Comparison of the effects of testosterone and dihydrotestosterone on the behavior of male cynomolgus monkeys (Macaca fascicularis)

To compare the behavioral effects of testosterone propionate (TP) and diyhdrotestosterone propionate (DHTP) at doses producing plasma levels of androgens within the physiological rage, observations were made on 4 castrated male cynomolgus monkeys during successive 4-week treatment periods while they received 25, 50, 100, 200, 400 and 800 μg of either TP or DHTP SC/day in counterbalanced order. Males were tested with each of the same 4 female partners (16 pairs, 1024 1-hr behavior tests). Males were injected at 1600 hr and blood samples were obtained at 0800 hr (256 samples, 456 hormone determinations). Physiological plasma levels of T resulted from the 200 μg and 400 μg TP treatments, and were associated with significantly increased ejaculatory behavior. Physiological plasma levels of DHT resulted from the 50 μg and 100 μg DHTP treatments, but there were no changes in ejaculatory behavior over the entire DHTP does range used. This difference in the behavioral effects of TP and DHTP, not previously reported for a primate, could not be accounted for by the effects of treatment order, season, long-term behavioral testing, female sexual motivation or behavior reflecting the peripheral action of androgens.

Effects of testosterone on the behavior of male cynomolgus monkeys ( Macaca fascicularis)

To determine the threshold doses of testosterone propionate (TP) that cause clear-cut behavioral changes in the sexual behavior of castrated male cynomolgus monkeys, observations were made on three males during successive 5-week treatment periods while they received daily subcutaneous doses of 100 μg TP increasing in octaves to 25.6 mg TP. Males were tested with each of the same two ovariectomized, estrogen-treated females (6 pairs, 330 1-hr behavior tests). To mimic the diurnal plasma testosterone rhythm, TP injections were given at 1600 hr and blood samples were obtained at 0800 hr (141 samples). Male ejaculatory activity increased at the threshold dose of 200 μg TP per day giving plasma testosterone levels of 830 ng/100 ml, which is in the physiological range of 600–1600 ng/100 ml for intact males. This threshold dose was eight times higher than in rhesus monkeys on a dose per kilogram body weight basis. There was a further marked increase in ejaculatory performance at higher doses (6.4 to 25.6 mg) giving supraphysiological plasma levels of 4000–9000 ng/100 ml. There were individual differences in the behavioral changes occurring with TP treatment, and the female partner modulated the effects. These findings were generally similar to those obtained with male rhesus monkeys, but certain species differences were noted.

Deciduomal response in androgenized rats: effect of age and steroid replacement therapy.

Deciduomal response was studied in female rats androgenized with a single injection of 1 mg of testosterone propionate at 5 days of age. Endometrial scratching in immature rats (33 days) elicited a better response in androgenized rats (AF) than in controls (NF) following induction of ovulation or steroid replacement therapy. In adult females receiving cervical stimulation at estrus or induction of ovulation, strong deciduomal response was obtained in NF rats and no response was observed in AF rats. In ovariectomized (OVX) rats receiving 2 mg of progesterone (P), the response in AF was only 50% that of NF rats. Addition of 0.1 mg of estradiol (E2) enhanced the decidualization in NF rats but completely abolished that of AF rats. Following ovariectomy and a period of 12-15 days without any exogenous hormone, an E2 priming treatment (0.2 or 0.5 micrograms) for 3 days followed by a replacement therapy (2.0 mg P + 0.1 or 0.15 micrograms E2) allowed good response in NF rats. The response was reduced by 30-35% in AF rats receiving 0.1 micrograms of E2 during the replacement therapy and by 66% in AF rats receiving 0.15 micrograms of E2. These results indicate that in AF rats the reduction of the response is age dependent, the uterus is more sensitive to E2 than is the uterus of NF rats and the growth response is always submaximal.

Neonatal exposure to DES in BALB/c male mice: Effects on pituitary-gonadal function

Neonatal male BALB/c mice were injected with diethylstilbestrol (DES), estradiol benzoate (E 2B), testosterone propionate (TP), progesterone or DES, in combination with E 2B, TP or progesterone and examined in adulthood. Body weight was reduced in males exposed to DES, TP or DES + TP, while testicular weight was reduced in animals injected with DES, E 2B, TP, DES + TP or DES + progesterone. Exposure to DES and/or E 2B also produced reproductive tract abnormalities and concomitant progesterone exposure did not further affect this parameter. Concomitant DES did not further alter the reduced plasma luteinizing hormone (LH) levels attributable to neonatal TP or E 2B treatment. Plasma follicle-stimulating hormone (FSH) levels in intact males were increased by DES, DES + progesterone or progesterone alone. Assessment of the feedback effects of exogenous gonadal steroids on pituitary gonadotropin release in castrated adults indicated that injection of 125 μg TP further increased the already elevated post-castration levels of LH and FSH in mice neonatally exposed to progesterone. The increase in testosterone (T) concentration after intratesticular human chorionic gonadotropin (hCG) administration was significantly attenuated in mice neonatally exposed to DES plus E 2B or to progesterone. Basal testicular T levels were significantly elevated in males exposed to DES, alone, or in combination with progesterone. Exposure to DES and TP increased hypothalamic serotonin (5-HT) levels in intact mice, while levels of 5-HT were lower after castration compared to controls. DES + E 2B-treated mice had higher norepinephrine (NE) levels, and E 2B-treated mice also had higher 5-HT levels. These findings indicate that alterations produced as a consequence of neonatal DES exposure include disruptions in pituitary-gonadal feedback and in brain biogenic amine levels, in addition to gross morphological abnormalities in the reproductive tract. Some of the effects of neonatal DES exposure were attenuated by simultaneous treatment with TP, E 2B or progesterone. In addition, treatment with these steroids was also capable of altering pituitary-gonadal feedback. However, gross genital abnormalities were apparent only in males receiving estrogenic preparations. Concomitant exposure to progesterone appeared to be capable of blocking DES-induced morphological effects.

Fluid Regulation and Reproductive Cyclicity in Female Rats Treated Neonatally with Testosterone and Methandrostenolone

Female rats injected with 1 mg of testosterone propionate on day 5 after birth weighed significantly more during the immediate postpubertal period than methandrostenolone-treated (1 mg) or vehicle-injected control females. There were no differences between groups in 24-hour intakes of food or water, when expressed on a per unit body weight basis. Testosterone- and methandrostenolone-treated rats ingested less water than controls in response to acute extracellular dehydration but not after cellular dehydration. The volume of the 'sexually dimorphic nucleus' of the preoptic area was significantly greater in brains taken from the two steroid-injected groups compared to control females. Testosterone had a stronger androgenic effect than methandrostenolone in terms of disrupting the estrous cycle.

Sex differences in the response to neonatal steroid treatment in the Mongolian gerbil

The present study examines whether genetic male and female gerbils differ in response to neonatal steroid treatment as evidenced by the display of sexually dimorphic territorial marking behavior in adulthood. The advantage of this behavioral endpoint is that male and female marking differs only in frequency, permitting direct between-sex comparison of neonatal treatment effects. Females and day-2 castrated males received a single SC injection of 10 to 100 μg testosterone propionate (TP) or estradiol benzoate (EB) between birth and day-14 postpartum. Between 7 and 9 months of age, the territorial marking behavior response to exogenous TP was determined in all animals. Based on marking frequency, the time of greatest sensitivity to TP administered during the critical neonatal period occurred earlier in the male (days 3–5) than in the female (days 5–7). In addition females consistently showed greater sensitivity to given dose of either TP or EB than did males. EB was more effective than TP at the 10 μg dose in both sexes. These results indicate that sex differences exist in timing and steroid sensitivity in the sexual differentiation of the neural substrate regulating territorial marking behavior.

The hormonal regulation of premeiotic steps of spermatogenesis in the newborn rat.

.he effect of high doses of testosterone propionate (TP) on the development of the first spermatogenic wave was systematically analyzed to determine the period of maximal susceptibility to testosterone. Two mg of TP were administered daily to groups of immature male rats, starting from birth, or days 5, 10, 15, and 20 until day 35 of life. Animals injected from birth or day 5 showed severe testicular atrophy, with reductions of more than 70% of testicular weight, diminished tubular diameters, and spermatogenic arrest during the meiotic prophase. Groups treated from days 10 or 15 showed increasing testicular weights with qualitatively normal spermatogenesis. When treatment started at 20 days, completely normal testicular development was achieved. To test the responsiveness of the neonatal hypothalamo-pituitary axis to TP administration, groups of 5-day-old male rats received daily injections of TP, and plasma FSH was determined at ten, 20, and 35 days. FSH levels were not detectable at ten and 20 days, and extremely depressed at 35. A group of 5-day-old male rats was injected simultaneously with 2 mg of TP and 14 IU of FSH (human menopausal gonadotropin: 71 IU of FSH and 80 IU of LH/ml) until day 35. Testicular weights and tubular diameters were increased compared to controls, and spermatid differentiation had proceeded to more mature steps than those seen in control animals. Inhibition of testicular development by neonatal TP administration was paralleled by a sharp decrease in circulating FSH levels and reversed by FSH replacement.(ABSTRACT TRUNCATED AT 250 WORDS)

Crowding or ACTH treatment of pregnant mice affects adult copulatory behavior of male offspring

Sexual behavior of male offspring from female mice chronically crowded during Days 12–17 of pregnancy was investigated. In an 80-min test pairing with a sexually experienced female primed with estradiol and progesterone injections, males from crowded mothers displayed poorer copulation than controls: mount and intromission latencies were longer, number of mounts and intromissions lower, and ejaculations within the test period were abolished. Daily injections of 500 μg testosterone propionate improved copulatory vigor in offspring from crowded mothers. A second series of experiments investigated the effects of ACTH treatment of females during the same period of pregnancy. A low dose rate (1 IU injected daily) had little effect but male offspring from females injected daily with 8 IU displayed longer intromission latency and fewer mounts and intromissions than controls. Daily injections of 500 μg testosterone propionate improved copulatory vigor, although mount frequency remained depressed. The similarity of the effects on male offspring copulation: of crowding their mothers during pregnancy or ACTH treatment during pregnancy suggest mediation by similar mechanisms, implicating involvement of maternal pituitary-adrenocortical secretions during pregnancy in the production of these behavioral deficits. Postnatal influences were minimized by fostering all litters at birth to untreated dams.

A study of the androgenic function of the epididymis

Rats were subjected to bilateral orchidectomy or orchido-epididymidectomy and maintained on either 500 μg testosterone or testosterone propionate daily. The ventral lobes of the prostate were subsequently excised and examined for androgen receptors in terms of the total present in the cytosol and the nucleus, the proportion unoccupied by endogenous androgen and the relative populations that were nuclease excisable or nuclease resistant in the two groups of animals. A further group of animals was subjected to unilateral deferential venotomy and the same parameters examined in the ipsi- and contra-lateral lobes of the ventral prostate and the seminal vesicles. In the absence of the epididymides there was a reduction in the number of receptors per prostatic cell and an increase in the proportion that were unoccupied. The nuclei from these glands contained fewer receptors than did those from the animals in which the epididymides had not been excised. The effect of unilateral deferential venotomy was to bring about a relative increase in the number of cytoplasmic receptors in the ipsilateral lobes of the ventral prostate with a much greater proportion unoccupied compared with the lobes contra-lateral to the ligation. There was again an increase in the proportion of nuclease-sensitive receptors in the nuclei ipsilaterally. The conclusions are that the absence of the epididymides in androgen-maintained rats or deferential venotomy induces a relative androgen-deficiciency of the prostate and seminal vesicles as reflected in the androgen receptor populations of these organs.

Conspecific urine marking in male-female pairs of laboratory rats

Male-female pairs of rats were observed during social interactions for conspecific markings where an animal deposits urine on the body of a second animal. Injections of sodium fluorescein were used to change urinary color and provide a visible mark on the back of a conspecific. The general hypothesis tested in the three experiments was that a female would mark males differently dependent upon her hormonal status and upon the relative hormonal integrity of the males. Results of Experiment 1 were that males marked females more frequently than vice versa and female marking decreased during her estrus. Moreover, a diestrous female marked an aggressive male more than she marked a non-aggressive male (Experiment 2), and a diestrous female marked a castrated male receiving 800 μg testosterone propionate (TP) injections more copiously than she marked a castrated male receiving 200 μg TP injections (Experiment 3). Finally, aggressive and non-aggressive males marked females similarly, though the 800 μg TP males marked females more than the 200 μg TP males. These data were compared with findings from research on environmental marking in rodents, and an hypothesis was suggested that female rats use conspecific marking to identify and select males with preferred behavioral and endocrine characteristics.

Studies of prostatic cytosol protein kinases of the aging ACI rat

Changes in prostatic cytosolic cyclic AMP (cAMP)-dependent and -independent protein kinases associated with aging in ACI rats were determined. A decrease in prostatic weight and tissue protein content (but not DNA) with age was noted. The cytosolic cAMP-dependent protein kinase was present predominantly as the type II isozyme. Total content of this enzyme per prostate declined by 37% in 12-month-old rats but remained relatively stable thereafer. However, the activity ratio (+cAMP/−cAMP) increased with aging, indicating an elevation in the amount of the holoenzyme or a decrease in the amount of the free catalytic subunit. Concomitantly the specific activity (activity/unit protein in the cytosol) of this enzyme was found to be increased; this is most likely due to the loss of bulk proteins in the cytosol, and may indicate a slower loss of this enzyme protein in relation to other cytosolic proteins. By comparison, the cAMP-independent protein kinase demonstrated a marked and progressive decline in amount as well as in specific activity with aging, e.g. 80–90% decline at 12 months of age. The above changes in these enzymes did not appear to be due to low circulating androgen in the animals, since they were not reversed on administration of 3.0 mg of testosterone propionate per rat daily for four consecutive days. It is therefore suggested that the above-described changes in prostatic cytosolic protein kinases are primarily associated with cellular senescence in this organ.

Induction of male sex behavior in pony mares with testosterone propionate

Two pony mares were administered 150 mg of testosterone propionate every other day for 20 days (ten injections) and every ten days there-after. An additional two mares and one stallion were not treated and served as controls. Testosterone propionate was dissolved in absolute ethanol and administered subcutaneously. Sex behavior tests were conducted 26 and 40 days after the first injection. Control mares exhibited very little male sex behavior. Both testosterone propionatetreated mares, however, exhibited mounting, sniffing, flehmen, biting and vocalization behavior in the presence of an estrous mare. The testosterone propionate-treated mares mounted and bit estrous mares more frequently than the stallion but exhibited less sniffing, flehmen and vocalization behavior in the presence of an estrous mare than the stallion. Testosterone propionate-treated mares and the stallion mounted an estrous mare 23.3 ± 9.7 seconds and 172.5 ± 22.5 seconds, respectively, after being introduced into the pen. Mares in estrus were mounted by the testosterone propionate-treated mares and the stallion an average of 4.0 ± 1.3 and 1.0 ± 0 times, respectively, during a ten-minute test. None of the non-estrous mares was ever mounted by the testosterone propionate-treated mares. In summary, testosterone propionate induced male sex behavior in intact mares and the testosterone propionate-treated mares effectively detected estrous mares.

Influence of androgen on the development of sexual behavior in the rat: II. Time and dosage of androgen administration during the neonatal period and masculine and feminine copulatory behavior in females

The objective of the present study was to delineate the period of sensitivity to a single androgen exposure during the initial neonatal hours on the development of masculine and feminine copulatory behavior in female rats. Female rats were injected once with either 500, 50, or 5 μg testosterone propionate (TP) at either 1 or 24 hr after birth. Following castration in adulthood and TP replacement, the females were tested four times at weekly intervals in prolonged sessions for masculine copulatory behavior. One month following the masculine copulatory tests the females were tested for 3 weeks for feminine copulatory behavior with weekly increasing levels of estradiol benzoate (2.5, 10, and 25 μg) and progesterone (200 μg). The results demonstrate that a single injection of TP administered at either 1 or 24 hr after birth can significantly increase the capacity of female rats to exhibit ejaculation patterns and that the amount of androgen that is administered is critical in determining the levels of ejaculatory responding. Similarly, the females given high doses (50 and 500 μg) of TP at either 1 or 24 hr neonatally were almost completely defeminized. In contrast, however, the females treated with 5 μg TP at 1 and 24 hr showed different levels of lordotic performance indicating a greater sensitivity to androgen immediately after birth than at 24 hr in female rats as has been shown in male rats.

Short- and long-term influences of neonatal sex steroids on α-bungarotoxin binding capacity in the mouse amygdala

Short- and long-term influences of various neonatal hormonal manipulations were assessed in the mouse amygdala with respect to α-bungarotoxin binding capacity, a possible parameter for cholinergic receptor integrity and function. To estimate the short-term effects, [ 125I]α-bungarotoxin binding to the tissue homogenate obtained from the posterior corticomedial amygdala was determined by a filtration assay using mice killed at 14 days post partum. The amygdaloid tissue from the normal male had a greater binding capacity for [ 125I]α-bungarotoxin than that from the normal female. However, castration of the male on the day of birth decreased the binding down to the female's level. A treatment of the female with either 250 μg testosterone propionate or 10 μg estradiol benzoate on days 1, 3, 5 and 7 increased the binding up to the male's level, although similar neonatal administration of 250 μg 5α-dihydrotestosterone was ineffective. The long-term effects of neonatal hormonal manipulations were examined with 77-day-old mice which had been gonadectomized for 49 days. A quantitative light-microscopic autoradiography for [ 125I]α bungarotoxin binding showed that specific grain density over the nucleus amygdaloideus medialis posterior of the androgenized female exceeded that of the female without neonatal hormone treatment. These observations provide further evidence that neonatal sex steroids play a determinate role in the sexual differentiation of the brain by exerting an organizational influence on developing cholinergic binding sites in the amygdala.

Prenatal testosterone exposure elevates maternal aggression in mice

Pregnant Rockland-Swiss (R-S) female mice were injected with oil, 0.5, 1.0 or 2.0 μg of testosterone propionate (TP) on days 12, 14 and 16 of gestation and the maternal aggressive behavior of their resulting female offspring was examined in adulthood. Prenatal exposure to 1 or 2 μg of TP, but not 0.5 μg of the steroid, significantly increased the number of attacks displayed by parturient mice toward adult male intruders. The behavioral effects on aggression were observed in the absence of effects on external morphology, body weight, or lactational performance. The findings support previous research showing that the development of feminine behavior may be sensitive to prenatal androgens. The possibility that the presence of fetal testosterone augments both male and female aggressive behavior is discussed.