['The purpose of this study was to examine the effects of voluntary wheel running on the positive reinforcing effects of heroin in rats with an established history of drug self-administration.', 'Rats were assigned to sedentary (no wheel) and exercise (wheel) conditions and trained to self-administer cocaine under positive reinforcement contingencies. Rats acquiring cocaine self-administration were then tested with various doses of heroin during daily test sessions.', 'Sedentary rats self-administered more heroin than exercising rats, and this effect was greatest at low and moderate doses of heroin.'] Does wheel running decrease the positive reinforcing effects of heroin?