Activation of the Raf and extracellular signal-regulated kinases (ERKs) (or mitogen-activated protein kinases) are key events in mitogenic signaling, but little is known about interactions with other signaling pathways. Agents that raise levels of intracellular cyclic adenosine 3 prime,5 prime-monophosphate (cAMP) blocked DNA synthesis and signal transduction in Rat1 cells exposed to epidermal growth factor (EGF) or lysophosphatidic acid. In the case of EGF, receptor tyrosine kinase activity and association with the signaling molecules Grb2 and Shc were unaffected by cAMP. Likewise, EGF-dependent accumulation of the guanosine 5 prime-triphosphate-bound form of Ras was unaffected. In contrast, activation of Raf-1 and ERK kinases was inhibited. Thus, cAMP appears to inhibit signal transmission from Ras by preventing Ras-dependent activation of Raf-1.