Rationale: Melanocortin and opioid systems regulate feeding as well as other behaviors; however, the relationship between the two systems is not yet defined. Since agonist-induced stimulation of melanocortin receptors blocks the behavioral effects of mu opioid receptor agonists, and melanocortin-4 (MC4) receptors and mu opioid receptors share a similar anatomical distribution in the central nervous system, MC4 receptor blockade may increase opioid responsiveness. Objectives: The goal of this study was to test the hypothesis that blockade of MC4 receptors increases the behavioral effects of morphine. Methods: The effects of HS014 (0.0032, 0.032, and 1 nmol, i.c.v.), a selective MC4 antagonist, on morphine-induced (3.2, 10, and 32 mg/kg, i.p.) locomotor activity (measured in the open field for 15 min) and antinociception (measured in the hot plate at 55°C) were assessed in C57Bl/6 mice. In addition, the effects of morphine were evaluated in Ay mice, a genetic model for MC4 receptor blockade. Results: The dose-effect curve of morphine for locomotor activity was shifted downwards in C57Bl/6 mice pretreated with HS014 and in Ay mice. The dose-effect curve of morphine for antinociception was shifted two- and threefold to the left in C57Bl/6 mice pretreated with HS014 and in Ay mice, respectively. Conclusions: These results indicate that blockade of MC4 receptors increases the antinociceptive effects of morphine without changing the potency of morphine for locomotor activity, suggesting that MC4 receptor antagonists may be candidate drugs that can be clinically used for the treatment of pain. © Springer-Verlag 2005.