In 14 healthy men we assessed the effects of smoking marihuana cigarettes containing 6 mg of delta-9-tetrahydrocannabinol on ultrasound measures of left ventricular function. Four of this group and four additional subjects also had measurements of plasma norepinephrine. Both heart rate and left ventricular performance (mean rate of internal diameter shortening [mean Vcf]) were significantly increased for at least 1 h after drug exposure compared with these values after placebo cigarettes. The immediate tachycardia and increase in mean Vcf were not accompanied by raised plasma norepinephrine levels. However, by 30 min after marihuana exposure, sympathetic neurotransmitter levels were significantly greater than both control values and those after placebo cigarettes, and they remained elevated for at least 2 h. Excessive sympathoadrenal discharge, as evidenced by augmented left ventricular function and prolonged catecholamine release, could adversely affect patients with heart disease.