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Recently, it has been appreciated that in addition to their antinociceptive properties, opioid analgesics also can enhance pain sensitivity (opioid-induced hyperalgesia [OIH]). OIH may enhance preexisting pain and contribute to dose escalation, tolerance, and misuse/abuse of opioids. Better information is needed to determine which opioid or opioid combinations may be least likely to produce OIH and therefore possibly represent better choices for pain management. Herein the authors have examined the hyperalgesic and antinociceptive properties of racemic methadone and its enantiomers alone and in combination with morphine in rats. Methadone is of particular interest because it possesses both μ-receptor agonist and N-methyl-d-aspartate receptor antagonist activities.The antinociceptive and hyperalgesic properties of d,l-methadone, l-methadone, and d-methadone were characterized by dose and sex using the thermal tail-flick test (high and low intensity). The responses to l- and d-methadone combinations with morphine were also determined with this model.Antinociceptive and hyperalgesic effects of d,l-methadone were demonstrated. These effects were related to dose but not to sex. The degree of hyperalgesia was greater with l-methadone compared with d,l-methadone. In contrast, d-methadone (N-methyl-d-aspartate antagonist) did not produce hyperalgesia. Furthermore, d-methadone blocked morphine hyperalgesia, enhanced antinociception, and abolished sex-related differences. This seems to be the result of antagonistic activity of d-methadone at the N-methyl-d-aspartate receptor.The current findings with methadone are supportive of previous findings implicating μ-opioid and N-methyl-d-aspartate receptor mechanisms in OIH. Better understanding of OIH may help in choosing the most appropriate opioids for use in the treatment of pain.