Since recent studies demonstrate that vascular smooth muscle cells synthesize two distinct guanylate cyclase-stimulatory gases, NO and CO, we examined possible regulatory interactions between these two signaling molecules. Treatment of rat aortic smooth muscle cells with the NO donors, sodium nitroprusside, S-nitroso-N-acetyl-penicillamine, or 3-morpholinosydnonimine, increased heme oxygenase-I (HO-1) mRNA and protein levels in a concentration and time-dependent manner. Both actinomycin D and cycloheximide blocked NO-stimulated HO-1 mRNA and protein expression. Nuclear run-on experiments demonstrated that NO donors increased HO-1 gene transcription between 3- and 6-fold. In contrast, NO donors had no effect on the stability of HO-1 mRNA. Incubation of vascular smooth muscle cells with the membrane-permeable cGMP analogues, dibutyryl cGMP and 8-bromo-cGMP, failed to induce HO-1 gene expression. Treatment of vascular smooth muscle cells with NO donors also stimulated the production and release of CO, as demonstrated by the CO-dependent increase in intracellular cGMP levels in coincubated platelets. Finally, incubating vascular smooth muscle cells with interleukin-1 beta and tumor necrosis factor-alpha induced NO synthesis and also significantly increased the level of HO-1 protein. The cytokine-stimulated production of both NO and HO-1 protein in smooth muscle cells was blocked by the NO synthase inhibitor methyl-L-arginine. These results demonstrate that exogenously administered or endogenously released NO stimulates HO-1 gene expression and CO production in vascular smooth muscle cells. The ability of NO to induce HO-catalyzed CO release from vascular smooth muscle cells provides a novel mechanism by which NO might modulate soluble guanylate cyclase and, thereby, vascular smooth muscle cell and platelet function.