Maturing amphibian oocytes undergo drastic morphological changes, including germinal vesicle breakdown (GVBD), chromosome condensation, and spindle formation in response to progesterone. Two kinases, maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK), are involved in these changes, but their precise roles are unknown. Unlike in Xenopus oocytes, discrimination of the functions of MAPK and MPF in Rana oocytes is easy owing to the lack of pre-MPF. We investigated the roles of these kinases by careful observations of chromosomes and microtubules in Rana oocytes. MPF and MAPK activities were manipulated by treatment with progesterone, c-mos mRNA, or cyclin B mRNA in combination with MAPK kinase inhibitors. Activation of one kinase without activation of the other induced only limited events; GVBD was induced by MPF without MAPK, and reorganization of microtubules at GVBD was induced by MAPK without MPF, but other events were not induced. In contrast, coactivation of MPF and MAPK by injection of c-mos and cyclin B mRNA promoted almost all of the morphological changes that occur during maturation without progesterone, indicating that these are controlled by cooperation of MPF and MAPK. The results revealed the functions of MAPK and MPF in each process of sequential morphological changes during oocyte maturation.