Ovarian and peripheral venous blood samples were collected at operation in six patients with polycystic ovaries (PCO) and seven control subjects with normal ovarian function in the follicular phase of the cycle. Plasma concentrations of unconjugated testosterone, dihydrotestosterone (DHT), androsterone, androstenedione, dehydroepiandrosterone (DHEA), pregnenolone, progesterone, 17 alpha-hydroxyprogesterone (17 alpha-OH-P) and oestradiol were determined by specific radioimmunoassay techniques, and concentrations of DHEA, androsterone and pregnenolone sulphates by gas chromatography. In the control group, ovarian vein concentrations of all unconjugated steroids, except progesterone, were significantly higher than the corresponding peripheral vein concentrations, suggesting ovarian secretion. No significant differences were demonstrated between ovarian and peripheral plasma levels of steroid sulphates. Ovarian vein levels of androstenedione, testosterone and DHEA were markedly elevated in PCO patients compared with the control group, and the levels of DHT and androsterone to a lesser degree. Polycystic ovaries appear to secrete androsterone sulphate, but no significant secretion of DHEA sulphate could be demonstrated. Ovarian venous levels of oestradiol in the PCO group did not differ from those in the control group. Elevated ovarian venous androgen levels in the PCO group seemed to correlate with thecal cell hyperplasia as indicated by histological examination of ovarian biopsies. In one PCO patient, blood levels of different steroids were followed for a month after ovarian wedge resection. Testosterone fell to half the pre-operative value and a temporary fall was also noticed in other androgens. A marked rise in plasma progesterone concentration at the end of the follow-up period suggested that ovulation had occurred.