p21ras plays an important role in the control of cell proliferation. The molecular mechanisms implicated are unknown. We report that the injection of oncogenic Lys12 Ras into Xenopus laevis oocytes promoted the activation of mitogen-activated protein kinase (MAP kinase) after a lag of about 90 min. MAP kinase activity was 10-fold higher 4 h after injection of oncogenic Lys12 Ras than after injection of nononcogenic Gly12 Ras. The stimulated MAP kinase activity remained at a plateau for at least 18 h. Maximal stimulation was obtained with 5 ng of Lys12 Ras, which is similar to the amount that elicits germinal vesicle breakdown. DEAE-Sephacel chromatography of extracts from Lys12 Ras-injected oocytes showed one peak of MAP kinase. MAP kinase activation by Lys12 Ras was associated with tyrosine phosphorylation of MAP kinase (p42). As previously shown, the S6-kinase II (likely pp90rsk), which is activated in vitro by MAP kinase, was also activated by oncogenic Lys12 Ras. Lys12 Ras with an additional mutation (Glu38) in the effector region that binds GTPase-activating protein (GAP) did not promote MAP kinase or S6 kinase activations. Thus, GAP may be involved downstream to Ras in these activation processes. Our results indicate that the Ras-GAP complex promotes MAP kinase activation in oocytes. This supports the idea that Ras-GAP controls MAP kinase, a kinase implicated in the action of various stimuli.