Mitogen-activated protein kinase (MAPK) (extracellular signal-regulated kinase) prevents DNA replication and parthenogenesis in maturing oocytes. After the meiotic cell cycle in starfish eggs, MAPK activity is maintained until fertilization. When eggs are fertilized, inactivation of MAPK occurs, allowing development to proceed. Without fertilization, highly synchronous apoptosis of starfish eggs starts 10 h after germinal vesicle breakdown, which varies according to season and individual animals. For induction of the apoptosis, MAPK should be activated for a definite period, called the MAPK-dependent period, during which eggs develop competence to die, although the exact duration of the period was unclear. In this study, we show that the duration of the MAPK-dependent period was approximately 8 h. Membrane blebbing occurred approximately 2 h after the MAPK-dependent period. Surprisingly, when MAPK was inhibited by U0126 after the MAPK-dependent period, activation of caspase-3 occurred earlier than in the control eggs. Thus, inactivation of MAPK is a prerequisite for apoptosis. Also, even in the absence of the inhibitor, MAPK was inactivated spontaneously when eggs began to bleb, indicating that inactivation of MAPK after the MAPK-dependent period acts upstream of caspase-3. Inactivation of MAPK also resulted in the activation of p38MAPK, which may contribute to apoptotic body formation.