Cyprodime (10 mg/kg, i.p.) selectively blocks MOR but not KOR according to tests of antinociception using the hot plate. Cyprodime pretreatment (1 h) blocked the antinociceptive effects induced by the MOR agonist morphine (10 mg/kg, i.p.) but not the KOR agonist U50,488 (10 mg/kg, i.p.). N = 8–16 per group. (*p < 0.05, ****p < 0.0001 compared to saline + vehicle, &p < 0.05 compared to saline + morphine).