Considerable effort has been devoted to identifying critical steps in mitogenic signal transduction pathways. Recently, the atypical PKC zeta isoform has attracted great interest since it has been reported to induce GVBD in Xenopus oocytes and transformation of NIH3T3 fibroblasts, two processes closely linked with the regulation of cell division. Furthermore, PKC zeta has been proposed as an essential effector for ras-p21 function and therefore may be an essential component of the signalling pathway(s) activated by mitogens. In this study we have analysed the responses induced in Xenopus oocytes after microinjection of purified recombinant PKC zeta protein. Microinjection of PKC zeta induced the early activation of MPF which precedes GVBD and also induced the activation of MAP kinase and S6 kinase II. The activation of MPF, MAP kinase and S6 kinase II by PKC zeta was sensitive to cycloheximide, while induction of GVBD was independent of protein synthesis. These results indicate that PKC zeta induces the activation of at least two pathways, only one of them leading to the activation of MAP kinase. By contrast, neither the induction of GVBD nor the activation of MPF, MAPK and S6 kinase II induced by the ras-p21 protein were dependent on protein synthesis. Thus, the comparison of these responses suggests that PKC zeta most likely does not mediate the ras-induced signal transduction pathway in Xenopus laevis oocytes.