The present study investigated the possible role of nitric oxide (NO) in the development of the withdrawal contractures of guinea pig isolated ileum after acute activation of mu- and kappa-opioid receptors. After a 4-min in vitro exposure to morphine (mu-opioid receptor preferring, but not selective, agonist), [D-Ala2-N-methyl-Phe4-Gly5-ol-]enkephalin (DAMGO; highly selective mu-opioid receptor agonist), or trans(+/-)-3,4-dichloro-N-methyl-N-2(1-pyrrolidynyl)cyclohexyl-ben zeneacetamide (U50-488H; highly selective kappa-opioid receptor agonist), the guinea-pig isolated ileum exhibited a strong contracture after the addition of naloxone. L-N(G)-nitro arginine methyl ester (3-300 microM) injected 10 min before the opioid receptor agonists was able dose dependently to reduce the naloxone-induced contraction after exposure to mu- and kappa-opioid receptor agonists whereas D-N(G)-nitro arginine methyl ester at the same concentrations did not affect it. The inhibitory effect of L-N(G)-nitro arginine methyl ester on morphine, DAMGO and U50-488H withdrawal was dose dependently reversed by L-arginine (3-300 microM) but not by D-arginine. Finally, glyceryl trinitrate on its own (3-300 microM) significantly increased the naloxone-induced contraction after exposure to mu- and kappa-opioid receptor agonist and it was also able to reverse the inhibition of opioid withdrawal caused by L-N(G)-nitro arginine methyl ester. These results provide evidence that NO has a role in the development of opioid withdrawal and that mu- or kappa-opioid receptors are involved.