The present study utilized an in vivo antisense strategy to examine the functional interaction of supraspinal mu-opioid receptors with the G protein subunits Gi1 alpha, Gi2 alpha, Gi3 alpha and Gs alpha. Mice were injected intracerebroventricularly (i.c.v.) with 33-base phosphorothioate oligodeoxyribonucleotides (12.5 micrograms) or with vehicle in equal volume (sterile water, 5 microliters) and the antinociceptive responses to i.c.v. morphine [D-Ala2,NMePhe4,Gly5-ol]enkephalin (DAMGO) or sufentanil were determined 18-24 h later using the tail-flick test. Treatment with antisense to Gi2 alpha, but not the other subunits, significantly attenuated morphine-induced antinociception. The degree of attenuation for the three mu-opioid agonists was in the order morphine > DAMGO > sufentanil, the inverse of their intrinsic efficacy.