A dose of 7 IU human chorionic gonadotropin (hCG) given 14 h before the expected LH peak on proestrus significantly increased embryonic mortality in Swiss random-bred female mice to 5 5% of the number of corpora lutea. The use of luteinizing hormone-releasing hormone in a similar injection protocol did not induce embryonic death. The effect found in Swiss random-bred mice resembles that of a dose of 20 IU hCG in the rat. Afternoon-day-4 mouse embryos contained 39.1 ± 12.6 nuclei after hCG-induced ovulation compared to 46.2 ± 16.6 nuclei after spontaneous ovulation. For early-day-5 embryos of the rat, these figures were 34.2 ± 10.1 and 31.7 ± 8.4, respectively (mating was early on day 1). Numerical chromosome errors were estimated in secondary oocytes of the mouse and early-day-5 embryos of the rat. Compared with data from the literature, hCG seems to induce some extra meiotic nondisjunction in the rat only. Combining all genetic and physiological data, the loss of fecundity after hCG-induced ovulation is a maternal effect.