The endogenous mos proto-oncogene product (Mos) is required for meiotic maturation. In Xenopus oocytes, the ras oncogene product (Ras) can induce meiotic maturation and high levels of M-phase--promoting factor (MPF) independent of endogenous Mos, indicating that a parallel pathway to metaphase exists. In addition, Ras, like Mos and cytostatic factor, can arrest Xenopus embryonic cell cleavage in mitosis and maintain high levels of MPF. Thus, in the Xenopus oocyte and embryo systems Ras functions in the M phase of the cell cycle. The embryonic cleavage arrest assay is a rapid and sensitive test for Ras function.