The purpose of the present investigation was to determine the effects of marihuana on physical performance as determined by six variables: heart rate, blood pressure, muscular strength, physical work capacity, forced vital capacity, flow rate of expiration. Twenty volunteer human marihuana smokers took part in three experimental testing sessions. Each subject reported to a laboratory on three different days. Day one was used as their control testing sessions. Day two and three were the placebo and marihuana sessions. On day three the subject smoked marihuana if he had placebo on day two, and vice versa. The test batter measures consisted of resting heart rate, resting systolic and diastolic blood pressure, grip strength, forced vital capacity and flow rate and a submaximal bicycle ergometer test. There were significant differences between the marihuana treatment group and the other treatment groups in heart rate, systolic and diastolic blood pressure and physical work capacity.