The effect of clomiphene citrate was tested on rat preovulatory follicles in culture. Clomiphene inhibited both basal and luteinizing hormone (LH)-stimulated steroid accumulation. The dose-dependent effect of clomiphene was much more pronounced on follicles cultured with LH; while 0.01 mM had no effect, 2 mM clomiphene decreased progesterone accumulation by 97%, estradiol-17 beta by 90% and testosterone by 65% (P less than 0.001). The inhibitory action of the drug was not prevented by addition of 8-Br-cAMP (1 mM) or pregnenolone (1 microgram/ml) to the culture medium. clomiphene (0.01 - 0.5 mM) induced the resumption of meiotic maturation even in LH-free medium in approximately 60% of follicle-enclosed oocytes. At the higher concentrations tested (0.1 - 0.5 mM) degenerative changes were noted in a high proportion of oocytes (30-40%). Follicles incubated for 24 h with clomiphene exhibited dose-dependent atretic-like changes. These effects of clomiphene on preovulatory follicles may account for the discrepancy between ovulation and pregnancy rates observed in women treated with clomiphene.