It is established that agmatine, an endogenously formed decarboxylated arginine, is a weak competitive inhibitor of neuronal nitric-oxide synthase (nNOS) with an apparent Ki value of 660 microM [Biochem J 316:247-249, 1996]. Although agmatine is known to bind to alpha-adrenergic and imidazoline receptors, it has been suggested that some of the pharmacological actions of agmatine, such as the prevention of morphine tolerance, may be due to the inhibition of nNOS. In the current study, we have discovered that agmatine, at concentrations much lower than the reported Ki value, leads to a time-, concentration-, NADPH-, and calmodulin-dependent irreversible inactivation of nNOS. The kinetics of inactivation could be described by an apparent dissociation constant for the initial reversible complex (Ki) and a pseudo first-order inactivation constant (k(inact)) of 29 microM and 0.01 min(-1), respectively. As determined by high-performance liquid chromatography analysis, the mechanism of inactivation involves alteration of the prosthetic heme moiety of nNOS, in part to protein-bound products. Moreover, we discovered that agmatine causes a 3-fold increase in the NADPH oxidase activity of nNOS leading to the production of H2O2 and is a likely cause for the inactivation of the enzyme. Both the inactivation of nNOS and the oxidative stress produced should now be considered in the pharmacological actions of agmatine as well as provide insight into the potential biological effects of endogenously formed agmatine.