To study the fertile life of acrosome-reacted guinea pig spermatozoa, spermatozoa were induced to undergo a synchronous acrosome reaction after which vigorously motile acrosome-reacted spermatozoa were isolated from acrosome-intact ones. Acrosome-reacted spermatozoa thus prepared were incubated in vitro for various periods of time before they were mixed with zona-intact and zona-free eggs. Such spermatozoa retained their ability to fertilize zona-intact eggs for at least 2 hours after the acrosome reaction; thereafter they rapidly lost this ability. There was a close correlation between the decline and loss of the ability to penetrate the zona and the loss of hyperactivated motility by spermatozoa, indicating the importance of vigorous motility in passing through the zona. The ability of acrosome-reacted spermatozoa to fertilize zona-free eggs was maintained for many hours even after the spermatozoa had completely lost their capacity to fertilize zona-intact eggs. As long as acrosome-reacted spermatozoa are "alive," they seem to retain the capacity to fuse with the egg plasma membrane regardless of the quality of their motility.