Loperamide (0.1–3.2 mg/kg i.p.) produced dose-dependent and complete suppression of writhing in the acetic acid-induced writhing assay in mice. Naltrcxone (NTX; 0.1–10.0 mg/kg s.c.) and its N-methylated derivative quaternary naltrexone (QNTX; 1.0 and 10.0 mg/kg s.c.) were roughly equipotent in antagonizing the antinociceptive effects of loperamide. In contrast, NTX was approximately 100-fold more potent than QNTX in antagonizing the antinociceptive effects of the classical mu agonist morphine. Furthermore, the antinociceptive effects of loperamide were not antagonized by central administration of the selective mu antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2(CTAP; 300 ng i.c.v.), or by systemic administration of either the kappa selective antagonist nor-binaltorphimine (nor-BNI; 32.0 mg/kg s.c.), or the delta antagonist naltrindole (NTI; 10.0 mg/kg s.c). These doses of CTAP, nor-BNI and NTI were effective antagonists of morphine, the kappa agonist U69,593 and the delta agonist BVV 373U86 [(±)-4-((R*)-a-((2S*5R*)-4-allyl-2,5-dimethyl-1-piperazinal)-3-hydroxybenzyl)-N,N-diethylbenzamide dihydrochloride], respectively. These results indicate that the antinociceptive effects of loperamide in mice are mediated, at least in part, by opioid receptors; however, these receptors are distinct from the opioid receptors mediating the effects of morphine, U69,593 and BW 373U86. These results are consistent with the hypothesis that loperamide produces its antinociceptive effects by acting, at least in part, at peripheral opioid receptors.