An attempt was made to directly examine the proposal that reinforcing effects result from activation of the nonspecific arousal system of the brain stem. Rats, with electrodes chronically implanted in the midbrain portion of the arousal system, were tested in a self-stimulation situation. Reinforcing properties of electrical stimulation of the arousal system were found to vary considerably from one rat the next. In some animals highly reinforcing effects were noted, in others only moderate effects, and in a third group no rewarding effects were observed. These results would seem to necessitate the postulation of a significant amount of functional localization within the nonspecific arousal system. (PsycINFO Database Record (c) 2006 APA, all rights reserved)