This study was conducted to examine follicle growth and intra-ovarian oocyte release in rabbit ovaries influenced by a local application of prostaglandin formation inhibitors. While five rabbits remained as unstimulated controls, twenty-seven mature rabbits were hyperstimulated by pure follicle stimulating hormone (FSH) followed by human chorionic gonadotropin (hCG) at time 0. At the same time, 1mg lonazolac (lonazolac group), indomethacin (indomethacin group) or saline (so-called gonadotrophic stimulated controls) was applied into the vagina of each group. Both ovaries of each animal were evaluated morphometrically at 15, 20, 28 and 48hr after hCG. The absolute numbers of smaller-sized preantral follicles were always higher in the gonadotrophic stimulated controls than in the unstimulated controls, the lonazolac, or the indomethacin group. The numbers of larger-sized preantral and antral follicles showed no differences. Unlike the gonadotrophic stimulated controls and the lonazolac group, the indomethacin group displayed an increase in its mature structures (large-sized antral and preovulatory follicles, follicle and luteinized cysts) from 20 to 28hr after hCG. Call-Exner bodies underwent pseudomucification in preovulatory follicles. The absolute numbers of Call-Exner bodies were augmented by indomethacin. Maturation division of oocytes began to decrease in all three treated groups between 15 and 20hr post hCG, but the height of intra-ovarian oocyte release (IOR) appeared at 28hr. This coincided with a striking thrombus formation. It is assumed that: 1) lonazolac and indomethacin inhibit early development of proliferating follicles; 2) lonazolac and indomethacin have no effects on the maturation division of the oocyte and of IOR; 3) IOR extends over a longer period after ovulation induction in contrast to the maturation division of the oocyte; and 4) thrombus formation is related to IOR.