The effect of the irreversible opioid receptor antagonist, beta-funaltrexamine (beta-FNA), on antinociception produced by mu- and kappa-receptor agonists was studied in the rat. beta-FNA, 20 to 80 mg kg-1, s.c., given 24 h before testing, produced a dose-related antagonism of the effects of morphine in the paw pressure, hotplate and tail-flick tests. Following the 80 mg kg-1 dose, the degree of antagonism of morphine was stable for up to 48 h after dosing, but was reduced by 5 days and had disappeared by 8 days. In the paw pressure test, beta-FNA, 40 mg kg-1, s.c., antagonized the effects of fentanyl, buprenorphine, tifluadom, ethylketocyclazocine and proxorphan; it was without effect against the highly selective kappa-agonist, U-50,488. In light of these results, the possible opioid receptor selectivities of both the agonists and beta-FNA are reassessed.