The etfects of Δ9‐tetrahydrocannabinol (Δ9‐THC) administered orally at 2 dose levels were studied in a group of 7 healthy young adult males. Each subiect was studied for 7 nights (2 drug, 5 placebo). Vital signs, subiective feelings, deep tendon reflexes, electrocardiogram, electroencephalogram, visual evoked responses, postural responses, time estimation and reaction time and sleep patterns were studied. At the doses studied, Δ9‐THC increased pulse rate, altered subjective feelings, and caused hyperreflexia and upset postural responses in the absence of visual cues. Some subjects also exhibited lowered oral temperature, changes in averaged visual evoked response, and alteration of sleep patterns.