The analgesic activities of a 75:25% nitrous oxide-oxygen mixture administered for 15 min, of delta-9-tetrahydrocannabinol (THC) 10 mg kg-1 i.p., and of a combination of both, were evaluated in the rat by tail-flick and hot-plate tests. The nitrous oxide-oxygen mixture produced a significant increase in the pain threshold. The analgesic activity of THC was similar in extent but of longer duration than that of nitrous oxide. The cannabinoid also induced some locomotor and behavioural modifications. When both THC and the nitrous oxide-oxygen mixture were administered, a significant potentiation of the analgesic response was produced, without modification of the locomotor and behavioural responses that were induced by THC alone. Such mixtures may prove of value in the control of chronic pain in man.