In order to evaluate the possible effects of delta-9-tetrahydrocannabinol (THC) on autonomic nervous system function, cardiovascular responses to intravenous isoproterenol, phenylephrine, atropine, and propranolol were compared in hospitalized volunteers before and after 14 days of THC ingestion. There was no significant alteration in responses to isoproterenol or phenylephrine, although in one subject the pressor effect of phenylephrine was considerably augmented by THC. Heart rates after parasympathetic (atropine) blockade and after combined parasympathetic and sympathetic (propranolol) blockade were significantly greater during THC ingestion. These and other data presented are consistent with the picture of reduced sympathetic and enhanced parasympathetic activity described in animals on THC. During THC ingestion, atropine had a pronounced pressor effect, which might represent a clinically significant drug interaction.