How do the p21v-ras proteins and their normal cellular counterparts regulate cell function? What is the molecular basis of action of these proteins? Biochemical, structural and functional similarities between the ras proteins and the vertebrate G proteins offer clues that may help to answer such questions. The G proteins couple a wide array of extracellular signals to regulation of a number of enzyme effectors, including adenylate cyclase, retinal cGMP phosphodiesterase and phospholipase-C. The RAS1 and RAS2 proteins of yeast regulate adenylate cyclase, whereas their close mammalian homologues, the p21ras proteins, do not. Both the ras and the G proteins are located at the cytoplasmic face of the plasma membrane and bind and hydrolyse GTP. Patchy amino acid sequence homologies between the two groups of proteins suggest a common evolutionary origin and common structural features, particularly in the GTP binding domain. In the GTP bound state both proteins are 'on' or activated, and each exhibits an intrinsic GTPase activity that turns off the active state. The analogies between the G and ras proteins suggest that the latter may also couple signal detector and enzymatic effector elements, and suggest strategies for identifying them.