Effects on airway dynamics, heart rate, and the central nervous system of various doses of Δ9-tetrahydrocannabinol administered in a random, double blind fashion using a Freon®-propelled, metered-dose nebulizer were evaluated in 11 healthy men and 5 asthmatic subjects. Effects of aerosolized Δ9-tetrahydrocannabinol were compared with aerosolized placebo and isoproterenol and with 20 mg of oral and smoked Δ9-tetrahydrocannabinol. In the normal subjects, after 5 to 20 mg of aerosolized Δ9-tetrahydrocannabinol, specific airway conductance increased immediately, reached a maximum (33 to 41 per cent increase) after 1 to 2 hours, and remained significantly greater than placebo values for 2 to 3 hours. The bronchodilator effect of aerosolized Δ9-tetrahydrocannabinol was less than that of isoproterenol after 5 min, but significantly greater than that of isoproterenol after 1 to 3 hours. The magnitude of bronchodilatation after all doses of aerosolized Δ9-tetrahydrocannabinol was comparable, but 5 mg of Δ9-tetrahydrocannabinol caused a significantly smaller increase in heart rate and level of intoxication than the 20-mg dose. Smoked Δ9-tetrahydrocannabinol produced greater cardiac and intoxicating effects than either aerosolized or oral Δ9-tetrahydrocannabinol. Side effects of aerosolized Δ9-tetrahydrocannabinol included slight cough and/or chest discomfort in 3 of the 11 normal subjects. Aerosolized Δ9-tetrahydrocannabinol caused significant bronchodilatation in 3 of 5 asthmatic subjects, but caused moderate to severe bronchoconstriction associated with cough and chest discomfort in the other 2. These findings indicate that aerosolized Δ9-tetrahydrocannabinol, although capable of causing significant bronchodilatation with minimal systemic side effects, has a local irritating effect on the airways, which may make it unsuitable for therapeutic use.