We studied the effects of NG-monomethyl-L-arginine (LMMA), a nitric oxide (NO) synthesis inhibitor, in canine intrapulmonary arteries constricted with phenylephrine. Isolated vessels were suspended in organ chambers containing modified Krebs solution, and isometric tensions were recorded. In contrast to the expected constriction predicted from other studies, LMMA instead caused dose-dependent vasorelaxation in phenylephrine-constricted canine pulmonary arteries. LMMA (1 and 10 microM) reduced the phenylephrine contraction by 5 and 23%, respectively. Similar dose-dependent relaxations were observed with NG-monomethyl-D-arginine (DMMA) but not with L-arginine (L-Arg), N epsilon-methyl-L-lysine, and another NO synthesis inhibitor, N omega-nitro-L-arginine (LNA), suggesting that a methyl group positioned at the guanidino-nitrogen is important in these responses. Vasorelaxation induced by LMMA and DMMA was not affected by pretreatment with the NO precursor L-Arg; however, responses were abolished by dissimilar cyclooxygenase inhibitors indomethacin (10 microM) and meclofenamate (2 microM). Pretreatment of vessels with LNA (100 microM) augmented LMMA-induced relaxations but attenuated DMMA-induced responses. LMMA- and DMMA-induced vasorelaxations were also observed in endothelium-rubbed vessels; DMMA-induced vasorelaxations were similar in vessels with and without endothelium, whereas LMMA-induced relaxations were increased in endothelium-rubbed vessels. These data suggest that LMMA and DMMA induced vasorelaxation by causing synthesis of dilator prostaglandins, which was independent of the presence of endothelial cells and the L-Arg-NO pathway. In addition, both endothelium removal and LNA pretreatment augmented LMMA-induced but not DMMA-induced relaxations, suggesting that inhibition of NO synthesis by LMMA opposes its cyclooxygenase-dependent vasorelaxant effects.