Marijuana is known to cause symptoms suggestive of orthostatic hypotension such as dizziness and fainting during upright posture. We examined changes in cerebral blood velocity (CBV) and peripheral circulation during upright posture after smoking marijuana in 10 right-handed male subjects with a previous history of exposure to marijuana. The participants were physically and mentally healthy and drug-free for a minimum of 3 months before the experiment. Middle cerebral artery CBV, blood pressure and pulse rate were recorded during reclining position and standing, before and after smoking a high-potency marijuana cigarette or a placebo cigarette administered during 2 separate visits to the laboratory. The participants were questioned about dizziness during the upright position. Six of 10 subjects reported moderate to severe dizziness during standing after marijuana but not placebo. Subjects who experienced severe dizziness during standing showed marked decreases in blood pressure and CBV. Those who reported moderate dizziness showed reduction in CBV but not blood pressure. Subjects who reported mild dizziness after marijuana and after placebo smoking showed minimal changes in blood pressure and CBV. Cerebral ischemia during upright position after marijuana smoking may be caused by hypotension and impaired cerebral autoregulation.