NG-Monomethyl L-arginine (L-NMMA) inhibits endothelium-dependent relaxation. It is widely accepted that this effect is due to the inhibition of L-arginine metabolism. Here we show that L-NMMA antagonizes the vasodilator effects of compounds other than L-arginine. In the endothelium intact rat aorta preparation, L-NMMA antagonizes the relaxation induced by N-alpha-benzoyl L-arginine ethyl ester, amiloride and dibutyryl cAMP. This effect of L-NMMA is not due to the inhibition of the basal release of endothelium-derived relaxing factor since relaxations of amiloride and dibutyryl cAMP are not significantly different in the presence or absence of endothelium. Addition of superoxide dismutase significantly attenuates the vasoconstrictor effect of L-NMMA and restores the endothelium-dependent relaxation, demonstrating that superoxide anion may be involved in the vascular effects elicited by L-NMMA. Paradoxically, at concentrations greater than 1 mM, L-NMMA elicits endothelium-independent relaxation, which is antagonized by methylene blue. The relaxation elicited by L-NMMA is accompanied by the formation of a citrulline-like product in the rat aorta. Thus, L-NMMA has properties other than simple inhibition of L-arginine metabolism in the rat aorta preparation.