Recent evidence has accumulated showing that activation of PLC-catalysed hydrolysis of phosphatidylcholine (PC-PLC) is a critical step in mitogenic signal transduction both in fibroblasts and in oocytes from Xenopus laevis. The products of ras genes activate PC-PLC, bind guanine nucleotides, have intrinsic GTPase activity, and are regulated by a GTPase-activating protein (GAP). It has been suggested that, in addition to its regulatory properties, GAP may also be necessary for ras function as a downstream effector molecule. In this study, evidence is presented that strongly suggests that the functional interaction between ras p21 and GAP is sufficient and necessary for activation of maturation promoting factor (MPF) H1-kinase activity in oocytes, and that PC hydrolysis is critically involved in this mechanism. Therefore, we identify GAP as a further step required for signalling through PC-PLC, and necessary for the control of oocyte maturation in response to ras p21/insulin but not to progesterone.