Serum inhibin levels were measured by RIA twice weekly for 4 weeks in 5 women with the polycystic ovary syndrome (PCOS). These were compared to those in 10 women with normal menstrual cycles. Serum inhibin levels were similar in the 5 PCOS women (mean, 199; range, 126-266 U/L) and were not significantly different from those in the normal women during the early follicular phase (227; 100-485 U/L) or midfollicular phase (243; 143-412 U/L) of their cycles. Inhibin levels were higher (P less than 0.001) in the late follicular phase (408; 227-732 U/L), at midcycle (623; 367-1058 U/L), and during the midluteal phase (1245; 898-1727 U/L) in the normal women compared to those in the PCOS group. Serum inhibin levels were also measured in PCOS (n = 8) and infertile (n = 14) women after the rise and subsequent diminished gonadotropin secretion that occurred during LHRH agonist administration. In both groups, serum LH and FSH increased after initiation of LHRH agonist administration; this increase was accompanied by parallel rises in serum estradiol and inhibin before suppression (PCOS women: r = 0.71; P less than 0.001; n = 108; infertile women: r = 0.42; P less than 0.05; n = 163). All hormone levels, including inhibin, decreased during continued LHRH administration. Five PCOS women underwent ovulation induction using combined LHRH agonist and human menopausal gonadotropin administration. Serum estradiol and inhibin rose in parallel in response to exogenous gonadotropins (r = 0.92; P less than 0.001; n = 77). In conclusion, we found no evidence of a primary defect in ovarian inhibin physiology in women with PCOS in terms of either basal or gonadotropin-stimulated (exogenous or endogenous) secretion.