Systemic extrogen therapy promotes multiple preantral follicular development in immature mice. Estrogen pretreated ovaries might therefore be a useful source of cells for in vitro studies of oocytes maturation. Silastic capsules (5.0 mm length; 3.18 mm outer diameter, 1.57 mm inner diameter) filled with diethylstilbesterol were implanted subcutaneously in experimental mice (ICR) for up to 6 days. Ovarian weight and histology in diethylstilbesterol pretreated and control animal were assessed before and after pregnant mare serum gonadotrophin treatment and after human chorionic gonadotrophin. The following results were obtained; 1. Ovarian weight was significantly increased by 6 days of diethylstilbesterol pretreatment. Subsequent ovarian weight gain in response to pregnant mare serum gonadotrophin and human chorionic gonadotrophin was increased. 2. Diethylstilnbesterol pretreatment stimulated the developed healthy preantral follicles. 3. Forty eight hours after pregnant mare serum gonadotrophin treatment, a larger number of the antral follicles which developed in diethylstilbesterol pretreated animals showed signs of atresia, whereas in the control ovaries there was a higher incidence of premature luteinization. 4. Forty eight hours after human chorionic gonadotrophin, numerous corpora lutea and occasional luteinized unruptured follicles were present in both control and diethylstilbesterol ovaries. 5. Ovulation rate, fertilization rate and subsequent preimplantation development in vitro were not adversely affected by diethylstilbesterol pretreatment. However, there was considerable variation in the ovulation rate the number of animals with more than 60 ovulations was greater in the diethylstilbesterol gorup (52.4%) as compared to the control (33.3%).