Agmatine, decarboxylated arginine, is a metabolic product of mammalian cells. Considering the close structural similarity between L -arginine and agmatine, we investigated the interaction of agmatine and nitric oxide synthases (NOSs), which use L -arginine to generate nitric oxide (NO) and citrulline. Brain, macrophages and endothelial cells were respectively used as sources for NOS isoforms I, II and III. Enzyme activity was measured by the production of nitrites or L -citrulline. Agmatine was a competitive NOS inhibitor but not an NO precursor. K i values were approx. 660 μM (NOS I), 220 μM (NOS II) and 7.5 mM (NOS III). Structurally related polyamines did not inhibit NOS activity. Agmatine, therefore, may be an endogenous regulator of NO production in mammals.