It has been proposed that the local renin-angiotensin system is activated in the adventitia after vascular injury. However, the physiological role of Angiotensin II (Ang II) in the adventitia has not been studied at a cellular level. This study was designed to assess the role of Ang II in the growth response of cultured adventitial fibroblasts (AFs). Adventitial explants of the rat thoracic aorta showed outgrowth of AFs within 5-7 days. Ang II caused hyperplastic response of AF cultures. The Ang II-induced mitogenic response of AFs was mediated primarily by the AT₁ receptor. Ang II caused a rapid induction of immediate early genes (c-fos, c-myc and jun B). Induction of c-fos expression was fully blocked by an AT₁ receptor antagonist but not by an AT₂ receptor antagonist. Epidermal growth factor (EGF), platelet-derived growth factor-BB (PDGF-BB) and basic fibroblast growth factor (bFGF) induced DNA synthesis in AFs. Co-stimulation of AFs with the growth factors and Ang II potentiated the incorporation of ³H-thymidine into DNA. Results from this study indicate that Ang II causes mitogenesis of AFs via AT₁ receptor stimulation and potentiates the responses to other mitogens. These data suggest that the Ang II may play an important role in regulating AF function during vascular remodeling following arterial injury.