[Phe1ψ(CH2-NH) Gly2 nociceptin-(1-13)-NH2, a pseudopeptide analog of nociceptin, was originally seen as an antagonist of nociceptin receptors. In the present study, it was observed that intracerebroventricular (i. c. v.) injection of this pseudopeptide (1, 5, 10μg) significantly decreased the tail-flick latency of rats, indicating a hyperalgesic effect, while intrathecal (i. t.) injection of it (1, 2.5, 10μg) dramatically increased the tail-flick latency, indicating an analgesic effect. This strengthened the in vivo evidence that [Phe1ψ(CH2-NH) Gly2] nociceptin-1-13-NH might be an agonist of nociceptin receptors.