There is very little information on the effects of long term usage of marihuana on man, and even less on moderate long term usage, such as that of the majority of youthful users. Twenty-nine male medical students who had smoked marihuana moderately and regularly for at least 3 years were compared on a large battery of sensitive neuropsychological tests with 29 students who had never used marihuana or other drugs. Essentially no difference between the two groups was found. Moderate social usage of marihuana in this stable group does not result in sufficient impairment of function that it can be detected by the most sensitive neuropsychological instruments now available.