Superoxide plays an important role in pulmonary arterial smooth muscle cell (SMC) proliferation and survival. The rapid reaction between superoxide and nitric oxide (NO) to form peroxynitrite suggests that endothelium-derived NO may influence adjacent SMC growth. To investigate this possibility, we determined the dose-dependent effects of NO on the proliferation and viability of pulmonary arterial SMC isolated from fetal lambs (FPASMC). Using fluorescence microscopy we found a dose-dependent decrease in superoxide levels in FPASMC treated with the NO donor spermine NONOate. This was associated with an increase in peroxynitrite-mediated protein nitration. At doses between 50 and 250 microM, spermine NONOate attenuated serum-induced FPASMC proliferation resulting in a G(0)/G(1) cell cycle arrest. This process involved a decrease in levels of cyclin A and an increase in the nuclear localization of p21 and p27. Furthermore, 500 microM spermine NONOate decreased viable cell number by inducing programmed cell death: FPASMC treated with 500 microM spermine NONOate displayed a loss of mitochondrial membrane potential, followed by caspase activation and DNA fragmentation. These data suggest that NO inhibits superoxide-induced proliferation of FPASMC and at higher doses induces apoptosis. NO donors may therefore prove to be useful therapeutic tools to treat diseases resulting from excessive proliferation of vascular smooth muscle.