Okadaic acid (OA), a specific inhibitor of protein phosphatases, induces a rapid activation (30 min) of MPF when microinjected into the Xenopus oocyte. Neither protein synthesis inhibitors nor cAMP counteract the action of OA. These results indicate that the inhibition of protein phosphatase(s) is sufficient for the in vivo activation of MPF even after the full activation of cAMP-dependent protein kinase. In all experimental conditions (plus or minus inhibitors of protein synthesis; normal or elevated cAMP levels) OA induces a burst of protein phosphorylation together with the activation of MPF. Cytological analysis shows that OA provokes the breakdown of the nuclear envelope, the depolymerization of lamin and the condensation of the chromosomes. However, no metaphase spindles are organized, indicating that inhibition of protein phosphatases strongly affects the function of the microtubule organizing center.