The p42 mitogen-activated protein kinase (MAPK) is required for progression through meiotic M phase in Xenopus oocytes. This report examines whether it also plays a role in normal mitotic progression. MAPK was transiently activated during mitosis in cycling Xenopus egg extracts after activation of the cyclin-dependent kinase Cdc2-cyclin B. Interference with MAPK activation by immunodepletion of its activator MEK, or by addition of the MEK inhibitor PD98059, caused precocious termination of mitosis and interfered with production of normal mitotic microtubules. Sustained activation of MAPK arrested extracts in mitosis in the absence of active Cdc2-cyclin B. These findings identify a role for MEK and MAPK in maintaining the mitotic state.