We investigated the antinociceptive effect of pentazocine hydrochloride (pentazocine) in combination with morphine hydrochloride (morphine) using two antinociceptive tests; i.e., the tail-immersion and scald-pain tests, in rats.In the tail-immersion test, the rat's tail was immersed in warm water at 47°, and the latency to a nociceptive response was measured.In the scald-pain test, the right hind foot was scalded by immersion into hot water at 57°.Two hours later, addnl. thermal stimulus was applied to the same foot, and the latency to a nociceptive response was measured.S.c. treatment with either pentazocine (6, 12, 24 mg/kg) or morphine (1.5, 3, 6 mg/kg) alone dose-dependently showed antinociceptive effects in both tests.The ED50 values (95% confidence limit) of pentazocine and morphine were 13.0 (5.4-31.5) and 2.4 (1.6-3.7) mg/kg in the tail-immersion test and 11.0 (4.5-26.6) and 3.8 (1.8-7.2) mg/kg in the scald-pain test, resp.Simultaneous treatment with pentazocine at the similar dose augmented the morphine (1.5 mg/kg)-induced antinociception, but did not diminish the morphine (6 mg/kg)-induced antinociception in both tests.These results suggest that the simultaneous administration of pentazocine at the antinociceptive dose and morphine exerts addnl. antinociceptive activity against thermal and scald-induced inflammatory pain.