Changes in the morphology and steroid content of ovaries were studied after 48 h of intravenous injection of 100 µg of cyproterone acetate or flutamide to diestrus or estrous rats. Treatment with cyproterone acetate at diestrus caused a decrease in the number of small follicles ( < 200 µm), freshly formed corpora lutea and the levels of estradiol-17 β in the ovary, suggesting inhibition of ovulation. Following flutamide administration at diestrus, the number of follicles at all stages of development were reduced with a concomitant decrease in the ovarian levels of the hormones. Thus, flutamide suppressed the growth and maturation of follicles. On administration of these drugs at estrous, the steroid content of ovaries was more pari passu with the increase in the number of mature and medium follicles. The differential effects of the two drugs are discussed in the light of these observations.