NG-Methyl-L-arginine (L-NMA) and NG-nitro-L-arginine (L-NNA) inhibited NO-induced cGMP accumulation in porcine aortic endothelial cells with half-maximally effective concentrations of 15 and 3.4 microM, respectively. The effects of both compounds were reversible, but the L-NNA-induced inhibition was only reversed by wash-out in the presence of 1 mM L-arginine. In short-term incubations (45 s) of membrane fractions, L-NMA and L-NNA exhibited similar potencies to inhibit endothelial NO synthase, but L-NNA was markedly more potent than L-NMA after prolonged incubation periods (> or = 3 min) due to induction of a pronounced, reversible enzyme inactivation.