Twelve regular marijuana smokers participated in a study designed to detect possible after-effects associated with marijuana smoking. Each subject was evaluated for two weekends-during one weekend they received only placebo marijuana (0.0% THC); the other weekend they received active marijuana (2.1% THC). Each weekend subjects received a total of 40 standardized puffs of marijuana smoke, administered during five separate smoking periods in the late afternoons and evenings. Each morning after smoking, subjects completed a series of questionnaires evaluating their sleep and mood, and then performed a battery of tasks to assess their psychomotor and cognitive function. Ratings of "high" and heart rate indicated that effective doses of THC were delivered to the subjects, and expired air carbon monoxide levels demonstrated effective smoke administration over the course of the weekends. No evidence of residual subjective intoxication was found, and most of the behavioral tasks and mood scales were unaffected the morning after. Statistically significant after-effects were obtained on a few measures, but with one exception, these were of negligible magnitude, inconsistent with previous findings, or likely artifacts of the experimental situation. In short, marijuana smoking was not associated with a "hangover" syndrome similar to those reported after use of alcohol or long-acting sedative-hypnotics.