Older mouse eggs apparently are ferilized more rapidly and develop more rapidly to the pronuclear stage than younger ones. After a low (1.5 IU) or a high (7.5 IU) dose of PMSG [pregnant mare serum gonadotropin], female mice were killed 13 or 17 h after hCG [human chorionic gonadotropin]. The eggs were mixed in vitro with preincubated spermatozoa and fixed 1-1 1/4 h later. Although fertilization levels were high in all groups, the stages of egg activation and sperm head decondensation differed significantly. The observed ranking, from most to least rapid fertilization, of eggs obtained from females treated with 7.5 IU and killed 17 h after hCG, 1.5 IU and 13 h, and 7.5 IU and 13 h, was consistent with the approximate length of time the eggs had resided in the oviduct, i.e., the longer that time period, the more rapid the fertilization. When eggs were fixed 4 1/4 h after miving with spermatozoa, the majority of older eggs were fully pronuclear while only a few of the younger eggs were as advanced, indicating accelerated nuclear development in the cytoplasm of the older eggs.