Ni2+, a toxic and carcinogenic pollutant and one of the leading causes of contact dermatitis, is shown to inhibit neuronal nitric oxide synthase (nNOS) in a competitive, reversible manner with respect to the substrate L-arginine (K-i = 30 +/- 4 muM). The IC50 values were dependent on calmodulin (CaM) concentration, but proved independent of Ca2+, tetrahydrobiopterin (BH,) and other essential cofactors, Ni2+ also inhibited CaM-dependent cytochrome c reduction, NADPH oxidation, and H2O2 production by nNOS, Overall, the action profile of Ni2+ was suggestive of an unusual, double-acting inhibitor of nNOS affecting L-arginine-binding and Ca2+/CaM-dependent enzyme activation.