Comparison of the plasma levels of steroids between ovarian and peripheral venous blood in polycystic ovarian disease revealed that polycystic ovaries secreted androstenedione, testosterone, dihydrotestosterone, dehydroepiandrosterone and 17-hydroxyprogesterone significantly more than healthy ovaries at the early follicular phase of the cycle, whereas the levels of estradiol and progesterone did not differ from the control values. Follicular cyst fluid from polycystic ovaries showed significantly higher concentrations of testosterone, androstenedione jand dehydroepiandrosterone and much lower levels of estrone, estradiol, progesterone and 17-hydroxyprogesterone than follicular fluid from healthy preovulatory follicles. Testosterone and estradiol did not, however, show any significant difference when follicles in polycystic ovaries were compared to those in healthy ovaries at early follicular phase of the cycle. According to these findings follicle development in polycystic ovaries is arrested at a stage corresponding to the early follicular phase of the cycle. Increased androgen and 17-hydroxyprogesterone secretion by polycystic ovaries seems to originate mainly from the hyperplastic theca interna cells.