Abstract:Background: The purpose of this study was to investigate the influence of single or chronic (21 days) administration of the serotonin and noradrenaline reuptake inhibitor, venlafaxine, on the antinociceptive action of the opioid receptor agonist, morphine, in streptozotocin (STZ)-induced hyperalgesia. Methods:The studies were performed on male Wistar rats. Changes in nociceptive thresholds were determined using mechanical stimuli. Diabetes was induced by a single administration of STZ (40 mg/kg, im). Results: Venlafaxine was shown to modulate analgesic activity of morphine in STZ-induced hyperalgesia. However, whereas acute co-administration of venlafaxine increased the analgesic activity of morphine, chronic treatment with venlafaxine attenuated opioid efficacy. Conclusion: Depending on the mode of administration (single or long-term), venlafaxine modulates analgesic activity of morphine. Further investigations are necessary to clarify the mechanisms of these interactions, which may be clinically relevant.