Acute pulmonary physiologic effects of smoked marijuana and oral Δ9-tetrahydrocannabinol were investigated in 32 healthy, experienced male marijuana smokers. After smoking of marijuana assayed at either 1 or 2 per cent Δ9-tetrahydrocannabinol, specific airway conductance increased immediately, reached peak levels at 15 minutes and was still significantly elevated at 60 minutes. In contrast, specific airway conductance decreased after both tobacco smoking and deep-breathing maneuvers that simulated marijuana smoking. Inhalation of 1250 μg of isoproterenol caused specific conductance to rise to less than 60 per cent of the average peak increase observed after 2 per cent marijuana. After ingestion of 10, 15 and 20 mg of Δ9-tetrahydrocannabinol in 12 subjects, specific airway conductance rose significantly as compared with placebo, attained peak levels three hours after ingestion and remained elevated for four to six hours. These findings indicate that both smoked marijuana and oral Δ9-tetrahydrocannabinol cause definite dilatation of the airways lasting as long as 60 minutes and six hours, respectively.