In the Xenopus oocyte, progesterone triggers M phase Promoting Factor (MPF) activation in a protein synthesis dependent manner. Although the synthesis of the p42 MAPK activator Mos appears to be required for MPF activation, p42 MAPK activity has been shown to be dispensable. To clarify this paradox, we attempted to activate the p42 MAPK pathway independently of Mos synthesis by cloning and using Xenopus H-Ras in the oocyte. We demonstrate that the injection of the constitutively active Xe H-RasV12 mutant induces p42 MAPK and MPF activation through two independent pathways. Xe H-RasV12 induces only a partial activation of p42 MAPK when protein synthesis and MPF activation are prevented. A full level of p42 MAPK activation is reached when MPF is activated and Mos is present. In contrast, MPF activation induced by Xe HRasV12 is achieved independently of Mos synthesis and p42 MAPK activation but still depends on protein synthesis. Therefore, the amphibian oocyte represents a new model system to analyse an original H-Ras pathway ending to MPF activation and distinct from the p42 MAPK pathway. The identification of the proteins synthesized in response to Xe H-RasV12 and required for MPF activation, represents an important clue in understanding the mechanism of progesterone action.