There has been little success in inducing a decidual reaction in the uteri of adult female rats given androgen neonatally. Burin, Thevenot-Duluc & Mayer (1963) reported a positive reaction after scratching the antimesometrial lining and giving progesterone (5 mg) daily for 8 days, but the incidence of success, the age of the animals, and the criteria for decidualization were not given. Barraclough (1967) reported that 25% of adult females given HCG to induce ovulation, and reserpine to sustain pseudopregnancy, produced a positive decidual reaction. All other reports are negative. The reasons for failure are not clear. Lack of oestrogen-binding capacity has been documented (McGuire & Lisk, 1969) and may play an important r\l=o^\le;the availability of progesterone receptors in the uteri of such females is unknown. In the present study, attempts were made to induce a decidual cell reaction in the uteri of adult females of various ages given different doses of testosterone propionate (TP) at 5 days of age. Holtzman strain female rats were given 50 \g=m\g, 250 \g=m\g or 1250 \g=m\g TP on the 5th day after parturition. A decidual reaction was induced in adult females by scratching the entire antimesometrial length of one uterine horn; the contralateral horn served as the control. A 300% increase in weight of the former over the latter was considered significant (DeFeo, 1963). Progesterone was administered subcutaneously dissolved in 0\m=.\1 ml sesame oil, as was oestradiol benzoate (OB) when it was used. Animals were killed on the 10th day after treatment began. Young adults were given progesterone for 9 days and were killed on the 10th day (Table 1 ). Uterine trauma on Day 6 elicited a response in all rats (Groups 2A and 3A). The need for progesterone was indicated by the lack of response with oil or with early withdrawal of treatment. The 2 mg dose of progesterone was used in all further tests. The progesterone apparently caused a reduction in gonadotrophin from the pituitary because ovarian weight was reduced (Groups 2A and 3A versus 4A and 5A). A positive response was obtained in about 25% of 3to 4-month-old females given 1250 μg TP neonatally (Table 1). Again, oestrogen did not increase the incidence of positive responses. The steroid treatment reduced the ovarian weight of the rats in three of the four groups. These results are similar to those