Inbred ACI Research Articles

Study on the effect of chemoembolization combined with microwave ablation for the treatment of hepatocellular carcinoma in rats.

We aimed to evaluate the combining effects of transarterial chemoembolization (TACE) and open local thermal microwave ablation in a hepatocellular carcinoma animal model. Tumor cubes were implanted into the liver of 30 male inbred ACI rats. Groups of 10 animals were treated at 13 days (TACE or microwave ablation) and 16 days (microwave ablation) postimplantation with combined therapy of TACE (0.1 mg mitomycin C; 0.1 mg iodized oil; 5.0 mg degradable starch microspheres) and microwave ablation (2450 Mhz; 45 s; 35 W) (study group A), TACE alone (control group B), or microwave ablation alone (control group C). At day 12 and day 25 tumor size was measured via magnetic resonance imaging and the relative growth ratio was calculated. Hepatic specimens were immunohistochemically examined for the expression of vascular endothelial growth factor (VEGF). Mean growth rates were 1.34±0.19 in group A, 3.19±0.13 in group B, and 4.18±0.19 in group C. Compared with control groups B and C, tumor growth rate in group A was significantly inhibited (P < 0.01). The VEGF-antibody reaction in peritumoral tissue (staining intensity at portal triad, percent antibody reaction and staining intensity at central vein) was significantly lower in group A compared with group B (P < 0.01). No significant difference between group A and group C could be observed. This investigation shows improved results of TACE followed by microwave ablation as treatment of hepatocellular carcinoma in a rat model, compared with single therapy regimen regarding the inhibition of growth rate and reduction of VEGF-level in peritumoral tissue.

Genetic etiology of renal agenesis: fine mapping of Renag1 and identification of Kit as the candidate functional gene.

Congenital anomalies of the kidney and urogenital tract (CAKUT) occur in approximately 0.5% of live births and represent the most frequent cause of end-stage renal disease in neonates and children. The genetic basis of CAKUT is not well defined. To understand more fully the genetic basis of one type of CAKUT, unilateral renal agenesis (URA), we are studying inbred ACI rats, which spontaneously exhibit URA and associated urogenital anomalies at an incidence of approximately 10%. URA is inherited as an incompletely dominant trait with incomplete penetrance in crosses between ACI and Brown Norway (BN) rats and a single responsible genetic locus, designated Renag1, was previously mapped to rat chromosome 14 (RNO14). The goals of this study were to fine map Renag1, identify the causal genetic variant responsible for URA, confirm that the Renag1 variant is the sole determinant of URA in the ACI rat, and define the embryologic basis of URA in this rat model. Data presented herein localize Renag1 to a 379 kilobase (kb) interval that contains a single protein coding gene, Kit (v-kit Hardy-Zukerman 4 feline sarcoma viral oncogene homolog); identify an endogenous retrovirus-derived long terminal repeat located within Kit intron 1 as the probable causal variant; demonstrate aberrant development of the nephric duct in the anticipated number of ACI rat embryos; and demonstrate expression of Kit and Kit ligand (Kitlg) in the nephric duct. Congenic rats that harbor ACI alleles at Renag1 on the BN genetic background exhibit the same spectrum of urogenital anomalies as ACI rats, indicating that Renag1 is necessary and sufficient to elicit URA and associated urogenital anomalies. These data reveal the first genetic link between Kit and URA and illustrate the value of the ACI rat as a model for defining the mechanisms and cell types in which Kit functions during urogenital development.

Stress, glucocorticoids and absences in a genetic epilepsy model

Although stress can alter the susceptibility of patients and animal models to convulsive epilepsy, little is known about the role of stress and glucocorticoid hormones in absence epilepsy. We measured the basal and acute stress-induced (foot-shocks: FS) concentrations of corticosterone in WAG/Rij rats, non-epileptic inbred ACI rats and outbred Wistar rats. The WAG/Rij strain is a genetic model for absence epilepsy and comorbidity for depression, which originates from the population of Wistar rats and, therefore, shares their genetic background. In a separate experiment, WAG/Rij rats were exposed to FS on three consecutive days. Electroencephalograms (EEGs) were recorded before and after FS, and the number of absence seizures (spike-wave-discharges, SWDs) was quantified.Both WAG/Rij rats and ACI rats exhibited elevated basal levels of corticosterone and a rapid corticosterone increase in response to acute stress. The WAG/Rij rats also displayed the most rapid normalization of corticosterone during the recovery phase compared to that of ACI and Wistar rats. FS had a biphasic effect on SWDs; an initial suppression was followed by an aggravation of the SWDs. By the third day, this aggravation of seizures was present in the hour preceding FS. This increase in SWDs may arise from anticipatory stress about the upcoming FS. Together, these results suggest that the distinct secretion profile of corticosterone found in WAG/Rij rats may contribute to the severity of the epileptic phenotype. Although the acute stressor results in an initial suppression of SWDs followed by an increase in SWDs, stress prior to a predictable negative event aggravates absences.

Carcinogenic activity of coltsfoot, Tussilago farfara l.

The carcinogenicity of young, pre-blooming flowers of coltsfoot, Tussilago farfara L., which is a herb of the tribe Senecioneae, family Compositae, and widely used as a herbal remedy, was studied in inbred strain ACI rats. Rats were divided into 4 groups: Group I received 32% coltsfoot diet for 4 days, and subsequently 16% diet until the termination of experiment. Groups II and III respectively received 8% and 4% coltsfoot diet for 600 days. Group IV was fed a normal diet as a control group. The experiments were terminated 600 days after the start of administration of coltsfoot diet. All the rats in Group I survived beyond 380 days after the start of feeding and 8 out of 12 rats developed hemangioendothelial sarcoma in the liver, whereas only one out of 10 rats developed the tumor in Group II, and no tumors were observed in Group III. Chemical studies on the dried, young flowers used in this experiment suggested that the carcinogenicity of coltsfoot is most probably due to senkirkine, a hepatotoxic pyrrolizidine alkaloid.

Non-MHC gene regulation of nerve root injury induced spinal cord inflammation and neuron death

Spinal ventral root avulsion leads to an inflammatory response around lesioned motoneurons and the subsequent degeneration of a large proportion of the neurons. We demonstrate here differences in the regulation of cytokine mRNAs, microglia/macrophage activation, MHC expression and nerve cell survival in the two inbred rat strains DA and ACI. These strains have similar major MHC haplotypes, but differ in their non-MHC background genes. T cells were rare in the lesioned segments and depletion of T cells did not affect the response. Thus, non-MHC gene(s) regulate the inflammation and neuron death after nerve trauma by mechanisms not involving antigen-specific immune responses.

Characterization of phenytoin-resistant kindled rats, a new model of drug-resistant partial epilepsy: comparison of inbred strains.

Previous work from our laboratory showed that amygdala-kindled Wistar outbred rats can be selected according to the increase of afterdischarge threshold (ADT) after phenytoin application. Animals that consistently do not respond to phenytoin (PHT) with an ADT increase (non-responders) are the first animal model of pharmacoresistant complex partial seizures. In this study, we determined the ability to respond to PHT in male kindled rats of different inbred strains. The experiments were performed in fully kindled rats of five different inbred strains, Wistar-Kyoto, Lewis, Fischer 344, ACI, and Brown Norway. The response type of each rat was revealed by four consecutive PHT applications (75 mg/kg, i.p.) in fully kindled rats. PHT application resulted in plasma concentrations ranging from some 16 microg/ml in Lewis rats to 35 microg/ml in Fischer 344 rats, and in slight ataxia, most strongly in Fischer 344 rats. The rats of each strain did not show a homogeneous response to PHT. A significant increase of ADT was found after 86-97% of applications in Lewis, Wistar-Kyoto, and Fischer 344 rats. In contrast, Brown Norway rats responded in only 34% of experiments. This led to a considerable number of responders (i.e., consistent ADT increase by >20%) in Fischer 344, Wistar-Kyoto, and Lewis rats. The only strain revealing nonresponders (i.e., consistent lack of ADT increase by >20% with PHT treatment) was Brown Norway. Inbred strains, although genetically more homogenous than outbred strains, differ in their response to PHT. Brown Norway rats can offer advantages for further detailed investigation of the resistance to PHT in the kindling model of complex partial seizures.

Prolongation of pig islet xenograft survival in rats immunosuppressed with FK506

FK506 a new and potent immunosuppressive agent has been shown to be effective in prolonging pancreatic islet allograft survival. The present study was to determine its efficacy in prolonging pig islet xenotransplantation in two different strains of rat recipients. A total of two dosages of FK506 at 1 or 2 mg/kg per day for 2 weeks and then at weekly intervals were tested as monotherapy for their effect on the survival of renal subcapsular xenografts of purified or impure adult pig islets in inbred ACI and outbred Wistar rats. Histological assessment indicated that FK506 at 2 mg/kg per day significantly prolonged purified pig islet xenograft survival and to 7.5 months in two of three ACI recipients. Monotherapy with a lower dosage of FK506 or transplantation with impure pig islets resulted in increased graft survival time over controls, but less than that with the 2 mg/kg per day FK506. The viable pig islet xenografts showed a normal appearance and were readily identified by immunohistochemical staining for insulin and glucagon and further confirmed by immunohistochemical staining with anti-pig islet specific monoclonal antibody clone P44, developed in our laboratory. Mononuclear cell infiltration, mainly of the CD8-positive T-cell subset, increased with the duration of the graft in the recipient. By 7.5 months the majority of the xenografted islet cells were enclosed by the cellular infiltrate. The in vitro perfusion study of pig islets that had survived for 1 or 2 months in vivo showed that they were responsive to glucose stimulation with increase in insulin secretion into the perfusate. The results demonstrated that FK506 significantly prolonged pig islet survival in two rat strains and suggested that it is an effective immunosuppressant for the xenotransplantation model.

Strain differences in the distribution of arginine-vasopressin- and neuropeptide Y-immunoreactive neurons in the suprachiasmatic nucleus of laboratory rats

We have studied the number of arginine-vasopressin (AVP)-immunoreactive (it) somata and the area size of AVP- and neuropeptide Y (NPY)-ir fibers in the suprachiasmatic nuclei (SCN) of three strains of laboratory rats exhibiting a strong unimodal (ACI), a bimodal (BH), and a weak multimodal pattern (LEW) of wheel running activity. In all three strains, AVP-ir somata and fibers were located predominantly in the dorsomedial SCN. Significant strain-differences were found for the area size of AVP-ir fibers as well as for the number and density of AVP-ir somata. The total number of AVP-ir somata was significantly higher in strain ACI (2238 ± 164) than in strains BH (1552 ± 137) and LEW (1426 ± 110), whereas the mean area of AVP-ir fibers was significantly larger in strain LEW (50 779 ± 2202 μm 2) than in strains ACI (39 034 ± 2095 μm 2) and BH (28 052 ± 1728 μm 2). Consequently, the density of AVP-ir somata was significantly lower in LEW rats, which have a weak multimodal activity pattern, than in BH and ACI rats, which have a bimodal and unimodal activity pattern, respectively. These data suggest that AVP neurons may be part of SCN output pathways controlling circadian activity rhythms. NPY-ir fibers have been identified mainly in the ventral part of the SCN. The mean area of NPY-ir fibers was smallest in BH rats (26 100 ± 1822 μm 2), which show a rather scattered activity onset, and larger in ACI (29 934 ± 2468 μm 2) and LEW rats (31889 ± 2728 μm 2), which have rather precise activity onsets. The inbred strains ACI, BH, and LEW may prove to be suitable models to further study distinct neuronal substrates of the SCN functionally correlated with characteristic parameters of circadian rhythms.

Genetics of spike-wave discharges in the electroencephalogram (EEG) of the WAG/Rij inbred rat strain: a classical mendelian crossbreeding study.

The WAG inbred strain might be an animal model for human absence epilepsy. To study the inheritance pattern of absence epilepsy, WAG rats were crossbred, in a classical Mendelian way, with inbred ACI rats which show no signs of epilepsy. In the parental strains, reciprocal F1 hybrids, F2, B1, and B2 generations, the number and duration of spike-wave discharges were determined. One hundred percent of the F1 animals showed spike-wave discharges, while the percentages for the F2, B1, and B2 generations were 79, 95, and 37%, respectively. These results suggest that the occurrence of spike-wave discharges is determined by one gene with a dominant mode of inheritance. Cavalli's least-squares fitting procedure suggested different genetic models for the two parameters (number and duration) during the two periods (dark and light). These results confirm our previous findings (Peeters et al., Behav. Genet. 20, 453-460, 1990) that a number of genes are involved in absence epilepsy. One dominant gene appears to determine the occurrence, however, while others manipulate the number and duration of epileptic phenomena during the two periods dark and light.

Cocaine toxicity: Genetic differences in cocaine-induced lethality in rats

Cocaine produces stimulation, depression, convulsions and death, and binds at several receptor sites. Thus cocaine may produce toxicity through similar or distinct pathways from those associated with stimulation. Genetic differences in stimulant response to cocaine have recently been reported for four inbred rat strains, ACI, F344, LEW and NBR. In the present study, significant genetic differences were found in cocaine-induced lethality, with a two-fold difference in LD 50 values seen between the most sensitive (NBR) and least sensitive (LEW) strains. Sensitivity to lethality was not correlated with sensitivity to stimulation, but was highly correlated with baseline activity. This lack of strong association between stimulant and lethal effects of cocaine may be important in clinical manifestations of the cocaine sudden death syndrome.

Genetics of absence epilepsy in rats

All rats of the WAG inbred strain show electrophysiological and behavioral phenomena reminiscent of human absence epilepsy. To study the genetic architecture of this kind of epilepsy, WAG rats were cross bred with inbred ACI rats which show no signs of epilepsy. Number and duration of spike-wave discharges per hour were determined from 24-h recordings of cortical EEG in parental strains and reciprocal F1 hybrids. All hybrids showed spike-wave discharges, indicating complete dominance for occurrence, but different genetic backgrounds were suggested for number and duration of the phenomena. These results imply that more than one gene is involved in absence epilepsy. Some genes determine the occurrence, while others may manipulate the actual number and duration of the epileptic phenomena.

Characterization of a cDNA for rat P-450g, a highly polymorphic, male-specific cytochrome in the P-450IIC subfamily.

Cytochrome P-450g (IIC13) is a highly polymorphic, male-specific rat liver isozyme which is a member of the P-450IIC subfamily. A cDNA, c5126 (1737 bp), for P-450g was isolated from a lambda gt11 library synthesized from (+g) male rat liver mRNA. Sequence analysis of the clone, c5126, revealed an open reading frame of 1473 nucleotides, which encodes for a 490 amino acid polypeptide possessing the 30 NH2-terminal residues reported for cytochrome P-450 (M-3) (P-450g) [Matsumoto et al. (1986) J. Biochem. 100, 1359-1371]. A high degree of sequence similarity (greater than 70%) exists between c5126 and the published sequences of cDNAs for members of the IIC subfamily, while its sequence similarity to other subfamilies (IA, IIB, and IIIA) was much lower (less than 55%). RNA blot analysis utilizing an oligonucleotide probe specific for P-450g revealed that P-450g mRNA was expressed in livers of male but not female Sprague-Dawley (CD) and ACI rats, indicating that the sex difference was regulated pretranslationally. Furthermore, expression of P-450g mRNA was age dependent in livers of male ACI rats (a homozygous, phenotypically high P-450g strain). However, the mRNA for P-450g was expressed equally in livers of outbred male CD rats representing either the high (+g) or the low (-g) phenotype and of inbred ACI rats (+g) representing the high phenotype, indicating that the defect in (-g) rats does not reflect differences in expression of P-450g mRNA.

IMMUNOLOGICAL STUDIES IN DIABETIC RAT RECIPIENTS WITH A PANCREATIC ISLET CELL ALLOGRAFT IN THE BRAIN

Intracerebrally (IC) transplanted outbred Wistar and inbred Lewis (AgB1/1) strain rat islets and pancreatic endocrine cells (PEC) were able to function for a prolonged period in nonimmunosuppressed diabetic inbred ACI (AgB4/4) rats across a major histocompatibility barrier. All recipients were sensitized to various degrees to the donor antigens, as demonstrated by circulating cytotoxic antibody, irrespective of the survival of the IC graft. Nevertheless, the antidonor antibody titers in the IC islet and PEC graft recipients were lower and peaked later when compared with ACI recipients that received an intraportal islet allograft. PEC were also transplanted IC in immunized ACI recipients. In recipients hyperimmunized by repeated splenocyte injections, accelerated PEC graft rejection was observed. In recipients with weaker immunization by intraportal whole islet allograft 2 months prior to the IC allograft, the IC PEC allografts were also rejected. To assess if ACI rats with long-term-functioning IC islet/PEC allograft developed tolerance to the donor antigens, these animals were transplanted with a donor-strain skin graft. The skin grafts were all rejected in a first-set fashion similar to normal control ACI rats. Also, 7/12 and 7/9 recipients rejected their functional IC islet or PEC allograft, respectively, following transplantation of a donor-strain skin allograft, thus indicating that the transplanted PEC maintained their antigenicity even after long-term survival of over 1 year in allogeneic recipients. The data indicate that the brain does possess immunoprotective properties for the islet/PEC allograft. The protection, however, is relatively weak and is possibly due to the paucity of the effector mechanism in the brain relative to that normally present systemically.

Mapping of genes for cytochromes P-450b, P-450e, P-450g, and P-450h in the rat. Demonstration of two gene clusters with P450-b,e localized on chromosome 1.

Inbred ACI, WF, and RCS rats having characteristic markers for albino (c), hemoglobin beta-chain (Hbb), and pink-eyed dilution (p) on chromosome 1 and expressing variants for hepatic cytochromes P-450b, P-450e, P-450g, and P-450h were used in genetic mapping studies for these hemoproteins. The results of WF X (ACI X WF)F1 and RCS X (WF X RCS)F1 backcrosses revealed the existence of two gene clusters designated the P450-b,e and P450-g,h loci. The linkage map P450-b,e-p-c-Hbb on rat chromosome 1 was demonstrated and found to be congruent with Coh(P450-b,e)-p-c-Hbb on mouse chromosome 7. P450-g,h is not linked with P450-b,e and the other markers tested on rat chromosome 1. It appears that close genetic linkage, rather than common functional/regulatory properties, typify members of cytochrome P-450 families/subfamilies.

Thioacetamide Hepatocarcinogenesis&lt;xref ref-type="fn" rid="FN2"&gt;2&lt;/xref&gt;&lt;xref ref-type="fn" rid="FN3"&gt;3&lt;/xref&gt;

Thioacetamide (TAA) was administered to inbred male ACI rats for 1 year to determine its hepatocarcinogenicity. The carcinogen was fed at a level of 0.035% in a semipurified diet so that other components could be manipulated. The following groups were used: group I, TAA alone; group II, TAA with riboflavin content at 0.002% (one-third of the daily requirement); group III, TAA plus a supplement of 0.05% crude bovine thyroid; and group IV, TAA, 0.002% riboflavin, and thyroid supplement. All groups demonstrated cirrhosis, neoplastic nodules, and cholangiofibromas to varying degrees. Group IV was invariably the most advanced in all lesions, while group II demonstrated the next most advanced picture of cholangiofibromas. These lesions persisted for 1 year after cessation of the diet. Primary hepatocarcinomas (PHC) were produced by groups I, III, and IV, with the highest number being in group IV. Group II evoked only primary cholangiocarcinomas (PCC). A number of both types of carcinomas transplanted successfully. Thus TAA appears to be a hepatocarcinogen capable of inducing either PHC or PCC, with the tumor type evoked somewhat dependent on the associated diet.

Experimental Induction of Uterine Cancer in Rats by N-methyl-N’-nitro-N-nitrosoguanidine

Two experiments were undertaken to induce uterine cancer in rats by intrauterine administration of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) in a total of 120 inbred female ACI rats, 5 weeks old. In Experiment I, 55 rats were divided into 3 groups and given a single injection of MNNG 50 mg (Group 1) or 100 mg (Group 2)/kg body weight emulsified in olive oil or olive oil alone (Group 3) into the uterine canals after the laparotomy. Uterine corpus tumors including endometrial adenocarcinomas were developed in 35.3% (Group 1) and 16.7% (Group 2) of the effective animals. Endometrial adenomatous hyperplasia which was considered to be a precancerous lesion, was observed in a few rats of Group 2. No tumors nor hyperplastic lesions were seen in the control group. (The incidence of tumors in the uterine corpus was significantly higher in the animals of Group 1 or 2 than in those of Group 3 (P less than 0.05]. In Experiment II, 65 rats were divided into 4 groups and treated as follows; Group 1: Rats were given intravaginal detention of absorbent cottons dipped with 0.5 mg of MNNG dissolved in 0.2 ml of olive oil for a week. The treatment was repeated 20 times once every two weeks. Group 2: Animals were treated by intravaginal detention of cottons containing 10% acetic acid or MNNG in alternate weeks 5 times each, and then the detention of MNNG-cottons 15 times every other week. Group 3: Rats were given the absorbent cottons containing olive oil alone 20 times biweekly.(ABSTRACT TRUNCATED AT 250 WORDS)

Heart and spleen "twin grafts" in rats: morphology and function in permanent survivals.

A new rat allograft model has been investigated for its potential to facilitate organ transplantation. In this procedure heart and spleen of male inbred LEW rat (RT1') were connected to the main abdominal vessels in inbred ACI rat (RT1a). With optimal surgical technique we obtain 64% permanent graft survival (5-14 months) in the splenectomized recipients. Cardiac histology ranged from preserved myocardium with moderate thickening of coronary artery walls to diffuse myocardial fibrosis with marked involutional changes of coronary arteries. Whereas palpation and EKG wrongly indicate good function beyond the fourth month, only the phonocardiogram shows the rapid functional loss which reflects the true morphologic involution.

DNA Content of Liver Cell Nuclei of N-2-Fluorenylacetamide-Induced Altered Foci and Neoplasms in Rats and Human Hyperplastic Foci2

The DNA content of cell nuclei of liver lesions induced in inbred male ACI rats by administration of 0.02% N-2-fluorenylacetamide (2-FAA) diet was measured with a microspectrophotometer in Feulgen-stained specimens. Altered liver foci of the eosinophilic, clear, and simple basophilic (toluidine blue weakly positive or negative) types had modal DNA values with a wider deviation (2C-10C) than the euploid pattern of normal hepatocytes (2C-6C). However, the DNA histograms of cells in hyperbasophilic (intensely toluidine blue-positive) foci were aneuploid (2C-18C) and resembled those of cells in hepatocellular carcinomas (2C-23C). Conversely, the modal DNA values of neoplastic nodules showed a narrower range (2C-12C). These results confirm that the hyperbasophilic foci may be a population similar to carcinoma in situ or may be a direct precursor lesion of hepatocellular carcinoma. In the livers from 4 humans, 3 of whom had a history of long-term use of some drugs, multiple hyperplastic foci resembling eosinophilic or clear cell foci in rats were identified. The modal DNA values of the cell nuclei in these foci showed a wider deviation (2C-10C) than in surrounding hepatocytes (2C-6C). In 2 patients, the lesions contained neoplasms and, therefore, hyperplastic foci in humans may be related to tumor development.

ラットにおける無アルブミン遺伝子座 (&lt;I&gt;alb&lt;/I&gt;) とhooded遺伝子座の連鎖

A linkage study of the analbuminemia (alb) with some coat color loci (a,b,c,h) has been carried out by crossing the analbuminemic random-bred mutant NAR strain with the abh tester strain of rats, the Agouti-Irish inbred ACI strain and the Agouti-Nonhooded inbred IS strain. The analbuminemia (alb) locus is not linked to a, b, and c loci but linked to hooded locus from F2 and backcross progeny of the F1 hybrids of abh X NAR, ACI X NAR and IS X NAR to NAR. Recombination value between alb and h was 16.4 +/- 2.3 per cent from the data of F2 and backcross progeny of (ACI X NAR) F1 and (IS X NAR) F1 to NAR. This is the third locus on the sixth linkage group of the rat.

Synergism of Estrogens and X-Rays In Mammary Carcinogenesis In Female ACI Rats&lt;xref ref-type="fn" rid="FN2"&gt;2&lt;/xref&gt;&lt;xref ref-type="fn" rid="FN3"&gt;3&lt;/xref&gt;

A previously demonstrated synergistic interaction between diethylstilbestrol (DES) and radiation on rat mammary carcinogenesis was extended to another estrogen, 17-ethinylestradiol (EE2). These newly reported results with EE2 demonstrated that the previously reported synergistic interaction between DES and radiation is not confined to just DES. Instead, these new results implied that the synergistic interaction is a synergistic interaction between the estrogenic activity of DES and radiation on rat mammary carcinogenesis. Female inbred ACI rats were used. By the end of the experiment, no neoplasia was detected in rats bearing cholesterol pellets, with and without X-ray exposure. No significant tumor data were obtained from rats treated with 0.1 me EE2, with and without X-rays. Approximately 50% of the rats treated with DES and approximately 90% of the rats treated with 1 mg EE2 had 1 or more mammary adenocarcinomas (MAC). X-rays synergistically increased the number of MAC per rat in the groups implanted with DES or 1 mg EE2. X-rays also increased the trend toward earlier increased incidence of rats with MAC as compared to rats treated with estrogens only. All rats treated with DES and 1 mg EE2 had pituitary tumors. The mean weight of the pituitary tumors in the groups treated with 1 mg EE2 was approximately 1.5 times that of the groups treated with DES. Mean terminal plasma prolactin levels for rats treated with 1 mg EE2 or DES were, respectively, 17.5 and 9.5 times control values.