The critical role of high level of serum LH in polycystic ovary syndrome was evaluated in rats by the long-term administration of hCG. Wistar-Imamichi strain mature female rats (age; 12 weeks) which showed at least two consecutive estrous cycles in vaginal smears were daily injected 10 IU hCG subcutaneously from diestrus for 80 days. Control rats were received saline solutions. On the next day after the last administration the rats were killed, and serum hormone levels and histological changes in the ovaries were examined. In 8 of 11 control rats the vaginal smears showed the regular estrous cycles (group 1) during the experimental period. None of the controls exhibited polycystic ovaries. In 19 of 25 experimented rats there were old corpora lutea and degenerated follicles (group 2). The remained animals (n = 6; group 3) showed polycystic ovaries and no corpora lutea except one. HCG treatment elevated the serum prolactin and estradiol levels in group 3, but reduced the progesterone level. Thus, it was suggested that the hCG-induced formation of PCO in rats might be able to refer to the pathogenesis in polycystic ovary syndrome.