Spermatozoa were recovered from oviducal ampullae 0.5 h after mating (the earliest sampling period), penetrated eggs were recovered at 1 h, and pronuclear stages at 4 h. Uterine spermatozoa maintained the initial high levels and vigor of motility until at least 4 h. Both the triple-stain and nonspecific esterase data indicated that more than 90% of such spermatozoa possessed intact acrosomes throughout the experimental period. Some loss of esterase activity was evident at 4 h (35% of acrosomes intact but esterase-negative). No motile spermatozoa lacking acrosomes were identified at any sampling interval in either uterine or ampullary preparations. These findings provide strong additional support for the view that, in the mouse, capacitation either does not occur or else requires less than 1 h. They also support the conclusion that a typical acrosome reaction does not accompany capacitation.