Delta9-Tetrahydrocannabinol (THC), the active component of marijuana, was studies to determine whether it might be useful for preanesthetic medication. Ten healthy subjects received THC intravenously in logarithmically spaced incremental doses. Four subjects received a total cumulatine dose of 135 mug/kg and four others, 201 mug/kg, Two of the ten subjects discontinued the study because of anxiety reactions. Ventilatory minute volume at a controlled elevated CO2 tension, 48 plus or minus 2 (SD) torr, changed minimally with TCH, -0.49 1/min/50 per cent increase in dose. TCH shifted the ventilatory response to CO22.7 torr destrad at 20 1/min without a change in slope. Dose-related tachycardia was the most marked cardiovascular effect. Heart rates increased to more than 100/min in five of six subjects. Cardiac index increased from 4.04 plus or minus 0.62 1/min/m-2 before TCH to 6.92 plus or minus 2.34 1/min/m-2 after 134 mug/kg. Mean arterial pressure increased slightly, and total peripheral resistance fell. The cardiovascular changes suggest beta-adrenergic stimulation. Intense mental effects and anxiety prohibited higher THC doses.