Mice were trained to avoid electric shock (0.6 mA) in a step-through type passive avoidance learning task, retention being measured 24 h after the training trial. Morphine 10 mg/kg administered 30 min before the test trial (pretest) facilitated memory retrieval, and the effect was completely antagonized by 1 mg/kg naloxone, a selective μ-opioid receptor antagonist. On the other hand, pretest administration of 0.01–10 mg/kg DTLET, a selective δ-opioid receptor agonist, did not produce the same effect as morphine. Nor-binaltorphimine, a κ-opioid receptor antagonist, did not antagonize the effect of pretest morphine, at doses of 1 and 2 mg/kg. These results suggest that the facilitation of memory retrieval by pretest morphine is mediated through μ- but not δ- or κ-opioid receptors.