To elucidate the process underlying polycystic ovary (PCO) syndrome, we tried to produce experimental PCO by sterilizing rats with androgen and investigated the formation of PCO endocrinologically and histologically. Wistar-Imamichi strain female rats were given an injection of testosterone propionate 1.25 mg on 5 days of age. On 5, 10, 20, 40, 60, 80, 100, 130, 160, 200 days after the injection the ovaries were obtained. Ovarian wedge resection was also performed at 100 and 200 day-rats. Serum gonadotropins, prolactin and steroids were measured with RIA and compared with ovarian pathology. Serum prolactin was increased drastically 40 days after the injection and maintained at a constant level higher than control. Through wedge resection, the serum LH level returned to normal, but FSH slightly increased. Serum estradiol decreased more markedly than estrone did. The 40 day-ovaries showed polycystic features, lack of corpus luteum, hyperthecosis, luteinization of stroma, while no thickening of tunica albuginea. In conclusion, the rats treated with testosterone propionate had many endocrinological and histological aspects similar to those of human PCO syndrome.