The EKG was continuously recorded in 10 normal subjects during marihuana and placebo administration. NIMH marihuana was administered in cigarettes calibrated to a delta‐9‐tetrahydrocannabinol content of 20 mg. Automated methods of data analysis were used to scan the EKG records for cardiac arrhythmias and to plot graphs of pulse rate and average T wave amplitude computed every 20 seconds over sessions lasting from 2 to 4 hours. Only 5 ventricular premature contractions occurred in all of the sessions; they were not related to marihuana intoxication. The marihuana‐induced tachycardia had a mean of 127.6 beats per minute with a latency of 17.8 minutes from the onset of smoking to maximum tachycardia. T wave changes were slight and did not correspond to other effects of marihuana. We suggest that syncopal episodes associated with marihuana intoxication are not associated with cardiac arrhythmias, but rather with orthostatic or vasovagal reactions combined with inhibition of compensatory vascular reflexes.