TRANSDUCTION of a mitogenic signal from the cell membrane to the nucleus involves the adapter proteins SHC and Grb2, which mediate activation of the Ras/mitogen-activated protein (MAP) kinase pathway(1-5). In contrast to receptor tyrosine kinases (RTKs), the signalling steps leading to Ras/MAP kinase activation by G-protein-coupled receptors (GPCRs) are still poorly characterized but appear to include beta gamma subunits of heterotrimeric G-proteins and as-yet unidentified tyrosine kinases(6-8). We report here that the epidermal growth factor receptor (EGFR) and the neu oncoprotein become rapidly tyrosine-phosphorylated upon stimulation of Rat-1 cells with the GPCR agonists endothelin-1, lysophosphatic acid and thrombin, suggesting that there is an intracellular mechanism for transactivation. Specific inhibition of EGFR function by either the selective tyrphostin AG1478 or a dominant-negative EGFR mutant suppressed MAP kinase activation and strongly inhibited induction of fos gene expression and DNA synthesis, Our results demonstrate a role for RTKs as downstream mediators in GPCR mitogenic signalling and suggest a ligand-independent mechanism of RTK activation through intracellular signal crosstalk.