Reproductive function following advanced ovulation induced by a single injection of 1 µg synthetic luteinizing hormone-releasing hormone (LH-RH) has been studied in rats. While advanced ovulation induced by an injection of the agent on the day of proestrus was followed by pregnancies occurring as frequently as those in spontaneous ovulation group, incidences of mating behavior, fertilization, and pregnancy were considerably reduced in the advanced ovulation group when induced by an injection of synthetic LH-RH during the early period of diestrus 2. The mean volume of primary oocytes of the three largest follicles at 1.00 h on the day of diestrus 2 was significantly smaller than that at 1.00 h on the day of proestrus. However, shortly before ovulation, the mean volume of secondary oocytes in the preovulatory follicles from rats treated with synthetic LH-RH at 1.00 h on the day of diestrus 2 was approximately the same as that in the preovulatory follicles from rats treated with the agent at 1.00 h on the day of proestrus. Shortly after ovulation the number of mitotic cumulus cells around the oviductal ova was significantly greater in advanced ovulation group induced by synthetic LH-RH injection during the early period of diestrus 2 than in spontaneous ovulation group. Furthermore, when examined 13 h after ovulation, the layers of cumulus cells were thicker, and cumulus cells were more tightly arranged around the ova in the advanced ovulation group induced by injection of synthetic LH-RH during the early period of diestrus 2 than those in the group with spontaneous ovulation. These results suggest that the cumulus-oocyte complex from advanced ovulation group induced by injection of synthetic LH-RH in the early period of diestrus 2 is less potent in fertilizability than that from spontaneous ovulation group, leading to a reduction in the rate of subsequent pregnancy.