In male R-3 mice alpha-endorphin (alpha-E) was injected intracerebroventricularly (icv, 10 ng) or intrathecally (ith, 1 or 10 ng). The antinociceptive activity (tail immersion test) and the locomotor activity (photocell actometers) were measured. alpha-E administered icv or ith induced an evident antinociceptive effect which was prevented by the previous naloxone injection. Locomotor activity was inhibited by icv but not ith administration of alpha-E. This effect was not changed by naloxone. The results show that exogenous alpha-E elicits different effects when applied icv or ith, and induces antinociceptive effect at the spinal level through opioid receptors.