The pharmacological activities of delta9-THC [(minus)-delta9-trans-tetrahydrocannabinol], CBN (Cannabinol) and mixtures of delta9-THC + CBN were studied in rabbits, rats and mice. CBN, although in general less active, mimicked the effects of delta9-THC in several pharmacological tests: corneal arreflexia in rabbits; climbing rope, open-field, irritability and aggressiveness after REM sleep deprivation in rats; catatonia, analgesia and sleeping time in mice. When the mixture delta9-THC + CBN was used, a synergistic effect occurred on most of the depressant effects. On the other hand CBN did not interfere with or slightly inhibited the excitatory effects of delta9-THC. In the one peripheral test used, CBN did not alter the delta9-THC effect.