We have investigated the effect of the NO synthesis inhibitor L-NG-nitro arginine (L-NNA) on the NANC relaxation induced by electrical field stimulation (FS) or the 5-HT1-like receptor agonist 5-carboxamidotryptamine (5-CT) in the longitudinal muscle of the guinea pig proximal colon. In the presence of atropine (0.2 microM) and guanethidine (5.0 microM), NANC relaxations induced by FS (0.1-10 Hz, 160 mA, 0.3 ms) or 5-CT (1.0-30 microM) were completely abolished by tetrodotoxin (0.3 microM). L-NNA (10-100 microM) produced concentration-dependent inhibition of NANC relaxations, but D-NNA had no effect. This inhibitory effect of L-NNA on NANC relaxations was prevented by L-arginine (1 mM) or L-citrulline (1 mM), but was not affected by D-arginine (1 mM). Like L-NNA, L-NMMA (100-300 microM) also inhibited the NANC relaxation induced by FS and 5-CT. These results suggest that nitric oxide (NO) or a substance releasing NO may contribute to NANC relaxations of guinea pig proximal colon induced by FS and the stimulation of 5-HT1-like receptors.