1. The hypothesis that endogenous tissue antioxidants might explain the inability of the superoxide generators 6-anilino-5,8-quinolinedione (LY83583) and hydroquinone (HQ) and of the NO-scavengers hydroxocobalamin (HC) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (c-PTIO) to affect nitrergic neurotransmission in the porcine gastric fundus was tested by selective pharmacological depletion of respectively Cu/Zn superoxide dismutase (Cu/Zn SOD) and reduced glutathione (GSH) in circular smooth muscle preparations. 2. Diethyldithiocarbamate (DETCA; 3x10(-3) M), which almost completely abolished tissue Cu/Zn SOD activity, had no effect per se on nitrergic relaxations induced by either electrical field stimulation (EFS; 4 Hz, 10 s) or exogenous nitric oxide (NO; 10(-5) M). In these DETCA-treated tissues however, electrically-induced nitrergic relaxations became sensitive to inhibition by LY83583 (10(-5) M) or HC (10(-4) M), but not by HQ (10(-4) M) or c-PTIO (10(-4) M); only for the combination of DETCA plus LY83583, this inhibition was partially reversed by exogenous Cu/Zn SOD (1000 u ml(-1)). 3. Immunohistochemical analysis of porcine gastric fundus revealed a 100% colocalization of Cu/Zn SOD and neuronal nitric oxide synthase (nNOS) in the intrinsic neurons of the myenteric plexus. 4. Buthionine sulphoximine (BSO; 10(-3) M) in the absence or presence of LY83583 (10(-5) M) or HC (10(-4) M) did not alter nitrergic relaxations, although it reduced per se the tissue GSH content to 62% of control. 5. Pharmacological depletion studies, corroborated by immunohistochemical data, thus suggest a role for Cu/Zn SOD but not for GSH in nitrergic neurotransmission in the porcine gastric fundus.