Intracerebroventricular (i.c.v.) administration of immune sera raised against Gi2 alpha subunits to mice, significantly reduced the supraspinal antinociceptive effect of opioids when evaluated 24 h later in the tail-flick test. Antisera directed against Gi1 alpha subunits did not modify this opioid activity. In mice injected with sera anti-Gx/z alpha, the mu-preferential agonists, DAMGO and morphine, and the endogenous mu/delta opioid peptide beta-endorphin-(1-31) displayed a reduced antinociceptive activity, whereas, the potency of the delta-selective agonists DPDPE and [D-Ala2]Deltorphin II, was not altered. This reduction was present for 3 to 7 days and returned to the control values after 10 days. Anti-Gi2 alpha and anti-Gx/z alpha, but not anti-Gi1 alpha, reduced the specific binding of [3H]DAMGO to the opioid receptor in PAG. These results suggest the ability of the mu receptor to interact in vivo with different classes of G transducer proteins (Gx/z/Gi2) to produce an effect. This work also indicates a functional role of the pertussis toxin insensitive Gx/z protein, on the mu-mediated (but not delta-mediated) supraspinal antinociception in mice.