Velocity of blood flow in the middle cerebral artery was measured with transcranial Doppler flowmeter before, during, and 1 hr after smoking a marijuana cigarette and a placebo cigarette during two separate visits to the laboratory. Ten healthy, right-handed male volunteers with a history of marijuana smoking took part in the study. The participants were drug-free for a minimum of 3 mo before the project. During the experiment, blood pressure, pulse rate, and end tidal levels of carbon dioxide were continually monitored. Marijuana smoking was associated with a significant increase in middle cerebral artery velocity. Although marijuana smoking was associated with increased pulse rate, the changes in blood velocity and pulse rate followed different time courses. Marijuana smoking was not associated with significant changes in blood pressure or end tidal carbon dioxide.