BACKGROUND: Peroxynitrite is a cytotoxic oxidant formed from nitric oxide (NO) and superoxide. Tyrosine nitration, a footprint of peroxynitrite, has been demonstrated in the pancreatic islets as well as in the cardiovascular system of diabetic subjects. Delineation of the pathogenetic role of peroxynitrite in disease conditions requires the use of potent, in vivo active peroxynitrite decomposition catalysts. The aim of the current work was to produce a potent peroxynitrite decomposition catalyst and to test its effects in rodent models of diabetes and its complications. METHODS: FP15 was synthesized and analyzed using standard chemical methods. Diabetes was triggered by the administration of streptozotocin. Tyrosine nitration was measured immunohistochemically. Cardiovascular and vascular measurements were conducted according to standard physiologic methods. RESULTS: FP15, a potent porphyrinic peroxynitrite decomposition catalyst, potently inhibited tyrosine nitration and peroxynitrite-induced cytotoxicity in vitro and in vivo. FP15 treatment (3-10 mg/kg/d) dose dependently and reduced the incidence and severity of diabetes mellitus in rats subjected to multiple low doses of streptozotocin, as well as in nonobese mice developing spontaneous autoimmune diabetes. Furthermore, treatment with FP15 protected against the development of vascular dysfunction (loss of endothelium-dependent relaxations) and the cardiac dysfunction (loss of myocardial contractility) in diabetic mice. FP15 treatment reduced tyrosine nitration in the diabetic pancreatic islets. CONCLUSIONS: The current results demonstrate the importance of endogenous peroxynitrite generation in the pathogenesis of autoimmune diabetes and diabetic cardiovascular complications. Peroxynitrite decomposition catalysts may be of therapeutic utility in diabetes and other pathophysiologic conditions.