When animals achieved a BP of FR25 or higher for food reward, they underwent the following surgeries: intracranial implantation of bilateral microinjection guide cannulae (described below) for the food pellet group, and both the intracranial implants and an IV jugular catheter for the cocaine group. Animals that attained higher than FR-25 were faster to acquire PR behavior after surgery. Food pellet group rats (N=12) were designated to return to food pellet reinforcer sessions, whereas cocaine group animals were (N=13) designated to receive IV cocaine self-administration training. An animal from each group was excluded from the experiment because in each case, their lever presses dropped to zero during the post-surgery sessions. Approximately five days after surgery, both food pellet and cocaine group animals were challenged first with FR-1 schedules on food pellets. The food pellet group was then challenged with PR 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
