Previous studies showed that the epidermal growth factor receptor (EGFR) can be transactivated by platelet-derived growth factor (PDGF) stimulation and that EGFR transactivation is required for PDGF-stimulated cell migration. To investigate the mechanism for cross talk between the PDGF β receptor (PDGFβR) and the EGFR, we stimulated rat aortic vascular smooth muscle cells (VSMC) with 20 ng of PDGF/ml. Transactivation of the EGFR, defined by receptor tyrosine phosphorylation, occurred with the same time course as PDGFβR activation. Basal formation of PDGFβR-EGFR heterodimers was shown by coimmunoprecipitation studies, and interestingly, disruption of this receptor heterodimer abolished EGFR transactivation. Breakdown of the heterodimer was observed when VSMC were pretreated with antioxidants or with a Src family kinase inhibitor. Disruption of heterodimers decreased ERK1 and ERK2 activation by PDGF. Although PDGF-induced PDGFβR activation was abolished after pretreatment with 1 μM AG1295 (a specific PDGF receptor kinase inhibitor), EGFR transactivation was still observed, indicating that PDGFβR kinase activity is not required. In conclusion, our data demonstrate that the PDGFβR and the EGFR form PDGFβR-EGFR heterodimers basally, and we suggest that heterodimers represent a novel signaling complex which plays an important role in PDGF signal transduction.