Conversion of external signals into proliferative responses may be mediated by interactions between signaling pathways that control cell proliferation. Interactions between G alpha(s), the alpha subunit of the heterotrimeric guanine nucleotide binding protein that stimulates adenylyl cyclase, and Ras, an important element in growth factor signaling, were studied. Expression of activated G alpha(s), in NIH 3T3 cells increased intracellular concentrations of adenosine 3’,5’-monophosphate (cAMP) and inhibited H-Ras-stimulated DNA synthesis and mitogen-activated protein kinase activity. Activated G alpha(s) and 8-Br-cAMP suppressed H-Ras-induced transformation of NIH 3T3 cells. Apparently, G alpha(s) inhibits proliferative signals from Ras by stimulating cAMP production and activating protein kinase A.