We have shown previously that intrathecal (i.t.) administration of the combination of delta 9-tetrahydrocannabinol (THC) and morphine results in a greater than additive antinociceptive effect. Similarly, pretreating mice with subthreshold doses of the kappa agonist, Dynorphin A (1-8), produced a parallel, leftward shift of the morphine dose-response curve, shifting the ED50 of morphine from 0.32 to 0.04 micrograms/mouse. A cocktail of enzyme inhibitors used to prevent the metabolism of Dynorphin A (1-8) into the delta receptor agonist, [Leu5]-enkephalin, attenuated the enhancement of morphine-induced antinociception by delta 9-THC. The enhanced antinociceptive effect observed after i.t. administration of the combination of delta 9-THC and morphine was also attenuated with antisera to Dynorphin A (1-8) (10 micrograms/ mouse) and Dynorphin A (1-13) (10 micrograms/mouse). Antisera to Dynorphin A (1-8) and Dynorphin A (1-17) blocked the antinociceptive effects of delta 9-THC (50 micrograms i.t.) without producing any significant alteration in the hypothermic and cataleptic effects or hypomotility produced by delta 9-THC. The antinociception produced by the combination of delta 9-THC and morphine was blocked by the kappa antagonist, nor-binaltorphimine (2 micrograms/ mouse), as well as the delta antagonist, naltrindole (5 micrograms/ mouse). Thus, the antinociception of morphine, which is mediated predominately by mu receptors, may be enhanced by delta 9-THC through the activation of kappa and delta receptors.