Using the latency of paw withdrawal (PWL) from a noxious thermal stimulus as a measure of hyperalgesia, the effects of i.p. injection of meptazinol and its isomers, 112824 and 112825, on carrageenan-induced thermal hyperalgesia were studied in awaked carrageenan-inflamed rats. Peripheral inflammation was induced by intraplantar (i.pl.) injection of carrageenan (2mg/100μl) into one hindpaw in rats. Carrageenan produced marked inflammation (edema and erythema) and thermal hyperalgesia in the injected paws, which peaked at 3 h after injection and showed little change in magnitude for another 3h. Injection of 0.1mg/kg meptazinol (i.p.) at 3h after carrageenan had no effect on the PWLs of either inflamed or non-inflamed hindpaw during the next 100 min (P>0.05, n=8). At the dosage of 1 and 10mg/kg, meptazinol produced marked anti-nociception and anti-hyperalgesia in non-inflamed and inflamed hindpaw, respectively (P<0.05, n=8～11). The prolonging effect of meptazinol on PWL in inflamed hindpaw was more potent than that in non-inflamed hindpaw. Pre-administration of 1.5mg/kg naloxone significantly antagonized meptazinol-induced anti-nociception and anti-hyperalgesia. Intraperitoneal injection of an isomer of meptazinol, 112825 (1.5mg/kg), but not 112824 (1mg/kg), markedly increased the PWL of the non-inflamed hindpaw. Nevertheles, both the isomers produced similar anti-hyperalgesic effect to that of meptazinol (P<0.05, n=8), which was completely reversed by naloxone (1.5mg/mg). The results suggest that meptazinol and its isomers have anti-nociceptive and anti-hyperalgesic properties with the former more potent. The effects are mainly mediated by mu opioid receptors. This study provides an important clue for extending clinical utilization of meptazinol and its isomers.