Abstract
Twentyโfour adult male research volunteers, in groups of three subjects each, lived in a residential laboratory for up to 18 days. All contact with the experimenters was through a networked computer system, and subject behavior was continuously monitored and recorded. During the first part of each day, subjects remained in private rooms doing planned work activities, and during the remainder of each day they were allowed to socialize. One or two cigarettes containing active marijuana (1.8โ2.7% ฮ^9^โTHC) or placebo were smoked during both the private work period and the period of access to social activities. Following the determination of baseline distributions of activities, contingency conditions requiring subjects to engage in a lowโprobability activity (instrumental activity) in order to earn time that could be spent engaging in a highโprobability activity (contingent activity) were then introduced. Baseline and contingency conditions were studied under periods of placebo and active marijuana administration. Under placebo conditions, introduction of the contingency resulted in increases in instrumental activity and decreases in contingent activity under both placebo and active marijuana conditions. The response to combining marijuana administration with contingencies varied across private work and social access conditions. During work periods, active marijuana administration increased instrumental activity to a larger extent than was observed under placebo conditions. The decreases in contingent activity were similar to those seen under placebo conditions. During social periods, active marijuana administration increased instrumental activity to a smaller extent than observed under placebo conditions. The decreases in highโprobability contingent activity were similar across drug conditions. Smoking active marijuana was thus observed to produce increments in instrumental activity under motivational involving contingencies for โwork activitiesโ while producing instrumental decrements under motivational conditions involving recreational contingencies.