Sex hormone status has emerged as important modulator of coronary physiology and cardiovascular disease risk in both males and females. Our previous studies have demonstrated that testosterone increases PKC expression and activity in coronary smooth muscle (CSMC). As PKC has been implicated in regulation of proliferation and apoptosis in other cell types, we sought to determine if testosterone modulates CSMC proliferation and/or apoptosis through PKC. Porcine CSMC cultures (passages 2-6) from castrated males were treated with testosterone for 24 hrs. Testosterone (20 and 100 nM) decreased [³H]-thymidine incorporation in proliferating CSMC to 59 ± 5.3% and 33.1 ± 4.5% of control. Flow cytometric analysis demonstrated that testosterone induced G₁-arrest in CSMC with a concomitant reduction in the S-phase cells. Testosterone reduced protein levels of cyclins D₁ and E and phosphorylation of retinoblastoma protein (pRb), while elevating levels of p21^cip1 and p27^kip1. There were no significant differences in the levels of cyclins D₃, CDK2, CDK4 or CDK6. Testosterone significantly reduced kinase activity of CDK2 and 6, but not CDK4, 7 or 1. PKC siRNA, prevented testosterone-mediated G₁-arrest, p21^cip1 upregulation and cyclin D₁, E downregulation. Furthermore, testosterone increased CSMC apoptosis in a dose dependent manner, which was blocked by either PKC siRNA or caspase-3 inhibition. These findings demonstrate that the anti-proliferative, pro-apoptotic effects of testosterone on CSMCs are substantially mediated by PKC.