—The signaling cascade elicited by angiotensin II (Ang II) resembles that characteristic of growth factor stimulation, and recent evidence suggests that G protein–coupled receptors transactivate growth factor receptors to transmit mitogenic effects. In the present study, we report the involvement of epidermal growth factor receptor (EGF-R) in Ang II–induced extracellular signal–regulated kinase (ERK) activation, c-fos gene expression, and DNA synthesis in cardiac fibroblasts. Ang II induced a rapid tyrosine phosphorylation of EGF-R in association with phosphorylation of Shc protein and ERK activation. Specific inhibition of EGF-R function by either a dominant-negative EGF-R mutant or selective tyrphostin AG1478 completely abolished Ang II–induced ERK activation. Induction of c-fos gene expression and DNA synthesis were also abolished by the inhibition of EGF-R function. Calmodulin or tyrosine kinase inhibitors, but not protein kinase C (PKC) inhibitors or downregulation of PKC, completely abolished transactivation of EGF-R by Ang II or the Ca²⁺ ionophore A23187. Epidermal growth factor (EGF) activity in concentrated supernatant from Ang II–treated cells was not detected, and saturation of culture media with anti-EGF antibody did not affect the Ang II–induced transactivation of EGF-R. Conditioned media in which cells were incubated with Ang II could not induce phosphorylation of EGF-R on recipient cells. Platelet-derived growth factor-β receptor was not phosphorylated on Ang II stimulation, and Ang II–induced c-jun gene expression was not affected by tyrphostin AG1478. Our results demonstrated that in cardiac fibroblasts Ang II–induced ERK activation and its mitogenic signals are dominantly mediated by EGF-R transactivated in a Ca²⁺/calmodulin-dependent manner and suggested that the effects of Ang II on cardiac fibroblasts should be interpreted in association with the signaling pathways regulating cellular proliferation and/or differentiation by growth factors.