The principal psycho-active constituent of cannabis, ∆-trans-tetrahydrocannabinol (∆-THC) has bronchodilator activity in man, both in normal subjects (Vachon, Fitzgerald, Solliday, Gould & Gaensler, 1973; Tashkin, Shapiro & Frank, 1973) and in asthmatic patients (Tashkin, Shapiro & Frank, 1974). It may be given orally, but in order to achieve significant effects, doses in excess of 10 mg must be used and patients then experience psychic disturbance. A dose of 200 ug, is an effective bronchodilator when delivered by inhalation from a pressurized aerosol, and no measurable systemic absorption takes place (Williams, Hartley & Graham, 1976). Higher doses frequently cause transient coughing and chest discomfort in asthmatic patients. The present study was undertaken to examine the effect of smaller doses of ∆-THC on ventilatory function, in terms of maximal response, duration and dose-response relation.