A number of excellent reviews dealing with many critical aspects of fertilization in mammalian and nonmammalian species have been published (Yanagimachi, 1978, 1981, 1982; Shapiro and Eddy, 1980; Longo, 1981a, b, 1985; Gulyas and Schmell, 1981; Wolgemuth, 1983; Wassarman, 1987). The review will address events of fertilization subsequent to the entry of the sperm nucleus into the oocyte cytoplasm, namely: the decondensation of the sperm nucleus, the formation of the male pronucleus, and the onset of DNA synthesis in the pronucleus. The focus will be on mammalian fertilization. However, events related to sperm nuclear decondensation and pronucleus formation and function in nonmammalian species also will be discussed as appropriate to support or extend what is known about mammalian fertilization. (Copyright (c) 1989 Academic Press, Inc.)