Nitric oxide (NO) released from endothelial cells or exogenous nitrates is a potent dilator of arterial smooth muscle; however, the molecular mechanisms mediating relaxation to NO in the microcirculation have not been characterized. The present study investigated the relaxant effect of nitrovasodilators on microvessels obtained from the rat mesentery and also employed whole cell and single-channel patch-clamp techniques to identify the molecular target of NO action in myocytes from these vessels. Both sodium nitroprusside (SNP) and S-nitroso-N-acetylpenicillamine (SNAP) relaxed phenylephrine-induced contractions by approximately 80% but were significantly less effective in relaxing contractions induced by 40 mM KCl. Relaxation to SNP was also inhibited by the K(+)-channel blocker tetraethylammonium or by inhibition of the activity of the guanosine 3',5'-cyclic monophosphate (cGMP)-dependent protein kinase (PKG). These results suggest that SNP stimulated K+ efflux by opening K+ channels via PKG-mediated phosphorylation. Perforated-patch experiments revealed that both SNP and SNAP increased outward currents in microvascular myocytes, and single-channel studies identified the high-conductance Ca(2+)- and voltage-activated K+ (BKCa) channel as the target of nitrovasodilator action. The effects of nitrovasodilators on BKCa channels were mimicked by cGMP and inhibited by blocking the activity of PKG. We conclude that stimulation of BKCa-channel activity via cGMP-dependent phosphorylation contributes to the vasodilatory effect of NO on microvessels and that a direct effect of NO on BKCa channels does not play a major role in this process. We propose that this mechanism is important for the therapeutic effect of nitrovasodilators on peripheral resistance and arterial blood pressure.