The purpose of the present investigations was to determine whether or not SIN-1, a metabolite of molsidomine that spontaneously releases nitric oxide, stimulates the production of adenosine-3',5'-cyclic monophosphate (cyclic AMP) and of guanosine-3',5'-cyclic monophosphate (cyclic GMP) in endothelial cells. All experiments were performed on first or second passage cultured porcine aortic endothelial cells. SIN-1 induced a time- and concentration-dependent accumulation of cyclic GMP but not of cyclic AMP. The production of cyclic GMP evoked by SIN-1 but not evoked by human alpha-natriuretic polypeptide was inhibited by treatment of the cells with either methylene blue (an inhibitor of soluble guanylate cyclase) and hemoglobin (a scavenger of nitric oxide). These data suggest that SIN-1 enhances the activity of soluble guanylate cyclase, which in turn induces the accumulation of cyclic GMP in endothelial cells. This response is probably due to the spontaneous release of nitric oxide, which is a potent activator of soluble guanylate cyclase.