Phosphatidylinositol 3-kinase (PI3K) is an important component of the signal transduction systems activated by tyrosine kinase receptors. It has not been established, however, whether PI3K is also an essential mediator for G protein–coupled receptors. The potential involvement of PI3K in G protein–linked angiotensin II (Ang II)–dependent signaling was assessed in a primary cell culture system of porcine coronary artery smooth muscle cells (SMCs). Treatment of quiescent SMCs with Ang II (10⁻⁵ to 10⁻⁸ mol/L) resulted in a dose-dependent activation of PI3K when assayed in vivo and in vitro. The Ang II receptor antagonists losartan and PD123319 were used to establish that Ang II stimulates PI3K through the Ang II type-1 (AT₁) receptor. Immunofluorescent microscopy revealed that Ang II (10⁻⁶ mol/L) stimulated the translocation of p85, the regulatory subunit of PI3K, from the perinuclear region to distinct foci throughout the cell within 15 minutes. Western blot analysis of p85 subcellular distribution demonstrated that p85 concentrations were also increased within 15 minutes in the membrane fraction and concomitantly decreased in the cytoskeletal and nuclear fractions. These changes in PI3K location and activity were paralleled by increased tyrosine phosphorylation of p85. A potential correlation between angiotensin-mediated PI3K activation and SMC growth was found using LY294002, a specific inhibitor of PI3K, which blocked the increase in DNA and RNA synthesis as well as cellular hyperplasia generated by Ang II (10⁻⁶ mol/L) stimulation of quiescent SMCs. These data indicate that PI3K may operate as a mediator of vascular SMC growth after stimulation with Ang II.