AbstractA marihuana extract, apomorphine, and amphetamine provoked in rats previously deprived of REM sleep for 96 hr an upright position with the animals pawing each other as in fighting. Nondeprived rats injected with the drugs did not show this aggressive behavior. The marihuana‐treated animals also bit vigorously a rod introduced into the cage, a fact not observed with the apomorphine‐treated rats. These data suggest that the marihuana aggressiveness could at least partially be mediated through the brain catecholamines. Haloperidol, chlorpromazine, α‐methyl‐p‐tyrosine, and reserpine decreased the marihuana aggressiveness of the deprived rats. Increase of dopamine concentration by previous treatment with Dopa potentiated aggressiveness. Blockade of norepinephrine synthesis by FLA‐63 was ineffective in reducing marihuana aggressiveness in the REM‐sleep‐deprived rats. Actually, in these animals aggressiveness appreared even without the marihuana injection. Phenoxybenzamine partially blocked marihuana aggressiveness but also severely depressed the rats. However, 20 mg/kg alone succeeded in provoking aggressiveness in the deprived rats. It is suggested that in rats with neural excitability heightened by previous REM‐sleep deprivation dopamine facilitates aggressive behavior, an effect inhibited by porepinephrine. This hypothesis would imply that marihuana induces aggressiveness in REM‐sleep‐deprived rats by inhibiting, through an as yet unknown mechanism, the central