The antinociceptive effect of intrathecally (i.t.) administered protease inhibitors was tested against capsaicin (800 ng) injected into the dorsal surface of a hindpaw. Both p-hydroxymercuribenzoate (2-8 nmol), a cysteine protease inhibitor, and phosphoramidon (1-4 nmol), an endopeptidase 24.11 inhibitor in the presence of bestatin (0.25 nmol) an aminopeptidase inhibitor, administered i.t. 60 min prior to the injection of capsaicin produced a dose-dependent reduction of the capsaicin-induced paw licking and biting response. p-Hydroxymercuribenzoate (4 nmol)-induced antinociception was significantly antagonized by nor-binaltorphimine, a selective kappa-opioid receptor antagonist, but not by naltrindole, a selective delta-opioid receptor antagonist. On the other hand, phosphoramidon (4 nmol) /bestatin-induced antinociception was significantly antagonized by naltrindole, but not by nor-binaltorphimine. The results indicate that the antinociceptive effect of p-hydroxymercuribenzoate may be due to the inhibition of a cysteine protease degrading endogenous dynorphins whereas phosphoramidon in the presence of bestatin blocks the degradation of enkephalins.