In order to investigate involvement of prostaglandins (PGs) in ovulation, it was examined whether inhibitors of cyclooxygenase, such as indomethacin and mefenamic acid, and the antiserum to cyclooxygenase were able to inhibit superovulation in immature female rats and gilts. Indomethacin and mefenamic acid were dose-dependently effective in inhibition of superovulation in the two animals. The antiserum to cyclooxygenase was raised in rabbits using the solubilized cyclooxygenase extracted and purified from bovine seminal vesicle as antigen. The in vivo administration of the antiserum inhibited superovulation in female rats dose-dependently at the doses of 0.1, 0.2 and 0.4 ml but did not influence production of progesterone. Inhibition of cyclooxygenase in the follicular cells by the antiserum with no effect on steroidogenesis was observed in organ culture of the follicles excised from the rats primed with PMS and the antiserum. On the contrary, no inhibition of cyclooxygenase was detected in case of the addition of the antiserum to the culture medium. The present study showed PG is one of the indispensable factors in ovulation as steroids are.