Twelve adult male research volunteers, in four groups of three subjects each, resided continuously in a residential laboratory for up to 18 days. Subject's behaviors, including social interaction, were continuously recorded. During the first part of the day (1000-1600), subjects remained in their private rooms doing work activities, and during the remainder of the day (1600-2345), they had the option to socialize with the other subjects. Four cigarettes containing active marijuana (1.84%, delta 9-tetrahydrocannabinol (delta 9 (THC)), w/w) or placebo were smoked daily: one each prior to the work and social access period, and two during the social access period. When the results were averaged across all groups and individuals, active marijuana significantly increased total daily social interaction time. However, drug effects were a function of the baseline amount of social interaction. The results of these experiments also showed that the pattern of increases in social interaction following smoked active marijuana varied within different groups of individuals.