schedules for food pellet rewards in sessions that were a maximum of 3 hr in duration; these animals tended to receive most of their rewards during the first hour of the session. The cocaine group was challenged with the PR schedule for cocaine infusions. Their IV catheters were connected to an infusion line that administered 0.75 mg/kg/infusion of cocaine over a 4 sec period. Infusions were accompanied by a 20 sec time-out (TO) period to avoid the potential for cocaine overdosing. Catheters were flushed with 0.2 cc saline, 0.1 cc Timentin, and 0.1 cc saline containing heparin immediately before animals were put into the operant chambers, and 0.1 cc saline containing heparin immediately after animals were taken out of the operant chambers. Sessions in the cocaine group lasted for a maximum of 4 hours since animals lever pressed for cocaine gradually and got most of their rewards by the third hour. Sessions for most of the cocaine group animals (N = 8) were conducted beginning 2 hours into the dark phase; however, for technical reasons, a smaller group (N = 5) was run beginning 2 hours into the light phase. Data were combined from the light and dark phase animals
