The i.c.v. administration of 0.5 microgram pertussis toxin to mice led to a non-competitive reduction (approximately 60 to 70%) of the supraspinal analgesia evoked by i.c.v. injection of ED90 doses of [D-Ala2,N-MePhe4,Gly-ol5]enkephalin, [D-Ala2,N-MePhe4,Met-(O)5-ol]enkephalin, [D-Ala2,Met5]enkephalinamide, [D-Ala2,D-Leu5]enkephalin or [D-Pen2,D-Pen5]enkephalin, whereas the analgesic effect of ED90 doses of morphine, etorphine, beta-casomorphin-(1-4) amide or human beta-endorphin was reduced to a lesser extent (about 20 to 30%). The co-administration of any of the opioids from the first group together with morphine resulted in antagonism of the effect elicited by the alkaloid. It is suggested that pertussis toxin treatment reduces differentially the efficacy displayed by various opioids when acting via mu receptors to produce supraspinal analgesia.