The roles of vasoactive intestinal polypeptide (VIP) and nitric oxide (NO) in nonadrenergic noncholinergic (NANC) nerve-mediated relaxations were investigated in longitudinal muscle strips of the rat gastric fundus. Transmural stimulation (1-16 Hz for 2 min), VIP and noradrenaline evoked a prolonged relaxation of the rat gastric fundus, whereas NO evoked a transient relaxation. Only the electrically induced responses were blocked by tetrodotoxin. The inhibitor of NO biosynthesis NG-nitro-L-arginine (L-NNA) preferentially inhibited the relaxations induced by low frequency stimulation. In contrast, trypsin mainly reduced the electrically induced relaxations to high frequency stimulation; the NANC relaxations resistant to trypsin were further inhibited by L-NNA. VIP-induced relaxations were abolished by trypsin, but remained unaffected by L-NNA. NO- or noradrenaline-induced relaxations were not inhibited by either L-NNA or trypsin alone, whereas the combination of L-NNA and trypsin slightly reduced the noradrenaline-induced responses. These results suggest that NANC responses in the rat gastric fundus at low frequency are mediated mainly by NO, whereas at higher frequency NO together with a peptide, probably VIP, are released.