We investigated the effects of pregnant mare serum gonadotropin (PMSG) and human chorionic gonadotropin (hCG) on ovarian carbonyl reductase activities towards 13,14-dihydro-15-ketoprostaglandin F2 alpha (15KD-PGF2 alpha), p-nitroacetophenone (PNAP) and p-nitrobenzaldehyde (PNBA) in mice and hamsters, and compared with their effects on those we observed previously in rats. The treatment with PMSG and hCG caused a significant increase in ovarian weights and superovulation in both mice and hamsters. Hamster ovary possessed appreciable carbonyl reductase activities towards all three substrates, whereas the activities were lower than those in rat ovary. The reductase activities were not increased by the treatment with gonadotropins, differing from rat ovarian carbonyl reductase. In untreated mice, carbonyl reductase activity towards 15KD-PGF2 alpha was not detected, whereas the activities towards PNAP and PNBA were detected, which activities were lower than those in rats and hamsters. The PNAP and PNBA reductase activities in mouse ovary were significantly increased up to 7.1- and 1.7-fold, respectively, by the treatment with gonadotropins. These results show that there are species differences in ovarian carbonyl reductase and response of the enzyme to gonadotropins.