3-Morpholinosydnonimine (SIN-1) is a metabolite of molsidomine which has been shown to decompose spontaneously in solution in the presence of oxygen. This reaction releases nitric oxide (NO) and superoxide anion, which under physiological conditions, react to produce peroxynitrite. SIN-1 is a potent vasodilator, activator of GCS (guanylyl cyclase), inhibitor of platelet aggregation, general cysteine protease inhibitor, and has been shown to produce significant protective effects in myocardial ischemia-reperfusion. Some of the effects of SIN-1 can be attenuated by nitroglycerin-induced tolerance.