To test the hypothesis that dihydroetorphine has opioid antagonist properties, we examined the effects of dihydroetorphine on the antinociceptive effect of morphine. The antinociceptive response was evaluated by recording the latency in the tail-flick test using radiant heat as a stimulus. The antinociceptive effect of dihydroetorphine reached its peak 30 min after administration, and remained significant up to 90 min after administration. On the other hand, when dihydroetorphine was injected i.c.v., the antinociceptive effect of dihydroetorphine reached its peak 15 min after administration, and a significant antinociceptive effect disappeared within 90 min after administration. When morphine was administered 30 min after dihydroetorphine, the antinociceptive effect of morphine was significantly enhanced. In contrast, the antinociceptive effect of morphine was significantly reduced when morphine was administered 120 min after either i.p. or i.c.v. administration of dihydroetorphine. This antagonistic effect remained significant up to 6 h after administration of dihydroetorphine, and then gradually decreased. However, dihydroetorphine had no significant effect on the antinociceptive effect of either trans-(+/-)-3,4-dichloro-N-methyl-N-(2(1-pyrrolidinyl) cyclohexyl)-benzeneacetamide (U-50, 488H), a kappa-opioid receptor agonist, or [D-Pen2.5]enkephalin (DPDPE), a delta-opioid receptor agonist. These results suggest that dihydroetorphine may have a reversible antagonist effect for mu-opioid receptors, but not for kappa- or delta-opioid receptors, when its agonistic activity disappears.