The induction of oocyte maturation is known to involve an initial action of agonists at the oocyte surface. This leads to activation of a cytoplasmic maturation-promoting factor (MPF) which induces the observable events associated with maturation (reviews by Smith and Ecker, 1970; Smith, 1975; Wasserman and Smith, 19786; Baulieu et al. 1978; Masui and Clark, 1979; Mailer and Krebs, 1980; Mailer, 1983; Gerhart et al. 1985; Masui and Shibuya, 1987). Recently, major advances have been made in the characterization of MPF and its mode of action in regulating cell cycle events. New evidence also has been obtained concerning the transmembrane signaling events that lead to MPF activation. The purpose of this article is to review and integrate these advances, partly in the context of earlier work in the field.