The natural arrest of vertebrate unfertilized eggs in second meiotic metaphase results from the activity of cytostatic factor (CSF).The product of the c-mosxe proto-oncogene is thought to be a component of CSF and can induce metaphase arrest when injected into blastomeres of two-cell embryos. The c-Mosxe protein can directly activate the mitogenactivated protein kinase kinase (MAP kinase kinase) in vitro, leading to activation of MAP kinase. MAP kinase and c-Mosxe are active in unfertilized eggs and are rapidly inactivated after fertilization. Microinjection of thiophosphorylated MAP kinase into one blastomere of a two-cell embryo induced metaphase arrest similar to that induced by c-Mos sup xe. However, only arrest with c-Mosxe was associated with activation of endogenous MAP kinase. These results indicate that active MAP kinase is a component of CSF in Xenopus and suggest that the CSF activity of c-Mosxe is mediated by MAP kinase.