We have previously reported that Tyr-D-Pro-Trp-Gly-NH_2 (D-Pro^2-Tyr-W-MIF-1) given spinally produces clearly a dose-dependent attenuation of the antinociception induced by Tyr-W-MIF-1 without affecting endomorphins- and[D-Ala^2, NMePhe^4, Gly(ol)^5]-enkephalin (DAMGO)-induced antinociception, and D-Pro^2-Tyr-W-MIF-1 at any doses used (0.025-1.2 nmol) does not show any antinociception or hyperalgesic effect by itself. In the present study, we found that D-Pro^2-Tyr-W-MIF-1 given supraspinally produced the antinociception, which is mediated by stimulation of μ-opioid receptors. D-Pro^2-Tyr-W-MIF-1 (0.5-16 nmol) given intracerebroventricularly (i.c.v.) produced an apparent dose-dependent antinociception. However, at the three highest doses (4, 8 or 16 nmol), there was a ceiling effect (about 30% MPE) where the increase in dose did not lead to a greater effect. The antinociception induced by D-Pro^2-Tyr-W-MIF-1 at a dose of 4 nmol was blocked by i.c.v. co-administration with the μ-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH_2 (CTOP), but not by i.c.v. pretreatment with the μ_1-opioid receptor antagonist naloxonazine, the κ-opioid receptor antagonist, nor-binaltorphimine, or the δ-opioid receptor antagonist naltrindole. In contrast, the antinociception induced by DAMGO and Tyr-W-MIF-1 was blocked by i.c.v. co-administration with CTOP or by i.c.v. pretreatment with higher doses of naloxonazine, but not by pretreatment with nor-binaltorphimine or naltrindole. We propose that the antinociception induced by D-Pro^2-Tyr-W-MIF-1 and Tyr-W-MIF-1 is mediated by the stimulation of different subtypes of μ_2-opioid receptors