Twenty-nine volunteers participated in a randomized, double-blind, placebo-controlled study. Cerebral blood velocity (CBV), pulse rate, blood pressure (BP), skin perfusion (SP) on forehead and plasma delta9 tetrahydrocannabinol (THC) levels were quantified during reclining and standing for 10 min before and after THC infusions and marijuana smoking. Both THC and marijuana induced postural dizziness, with 28% reporting severe symptoms. Intoxication and dizziness peaked immediately after drug. The severe dizziness group showed the most marked postural drop in CBV and BP and showed a drop in pulse rate after an initial increase during standing. Postural dizziness was unrelated to plasma levels of THC and other indices.