['Carbon monoxide (CO), like nitric oxide (NO), stimulates soluble guanylyl cyclase and thereby raises intracellular levels of cGMP. We examined the endogenous capacity of vascular smooth muscle cells (SMCs) to produce CO from heme through the activity of heme oxygenases.', 'Cultured SMCs from rat aorta (RASMCs) expressed immunoreactive inducible heme oxygenase-1 (HO-1) and constitutive HO-2. Treatment of RASMCs with hemin and sodium arsenite, which are inducers of HO-1, stimulated RASMC cGMP without stimulating nitrite release or inducible NO synthase expression, and the induced elevations of cGMP were not inhibited by the NO synthase inhibitor NG-methyl-L-arginine. Induced CO from RASMCs likewise caused elevation of cGMP levels in platelets coincubated with the vascular cells. Zinc protoporphyrin IX, an inhibitor of HO, reversed the inducible increases in platelet cGMP.'] Do vascular smooth muscle cell heme oxygenases generate guanylyl cyclase-stimulatory carbon monoxide?