To establish the role of hormone receptors in patients with polycystic ovary (PCO), PCO rats were prepared by treating with testosterone propionate (TP). Five-day old immature female rats were subcutaneously injected with 1.25 mg TP in sesame oil. They were then killed at the age of 12 weeks. The ovarian receptors for LH and FSH as well as serum hormone levels were investigated in PCO rats and also in control rats at the various stages of the estrous cycle. The LH receptor binding in the TP-treated ovaries was elevated almost as high as that of proestrus control, and was observed to be higher than the other control. The FSH receptor binding of PCO rats was elevated to 173% of that of diestrus control, which showed the highest value throughout the cycle. Thus, the gonadotropin receptors in PCO rats appeared to be in an activated state. High levels of the receptor binding were due to an increase in receptor binding sites. Serum LH level was significantly higher than that of diestrus control but still remained lower than that of proestrus control. In contrast, FSH level was as low as that of diestrus control. Prolactin level was markedly elevated and 17- and 2-fold higher than that of diestrus and proestrus control, respectively. Estradiol level was higher than that of diestrus control, increasing to almost the same level during proestrus control. While progesterone level was largely depressed to 23 and 13% of that of diestrus and proestrus control, respectively, testosterone level was almost the same as that of diestrus control. From these results, it was suggested that tonic secretion of LH, low level of FSH, and markedly high levels of prolactin would increase the gonadotropin receptors and result in extremely low production of progesterone in rat ovaries. Clinically, elevated levels of the LH and FSH receptors may be a relevant occurrence in PCO patients.