The endocannabinoid system plays a role in obesity, primarily by its role in food reward. Activity, also involved in obesity, seems to be at least partially controlled by the endocannabinoid system, but the relevant behavioral and neurochemical mechanisms have not been well established. This study represents an attempt to begin elucidating these mechanisms by examining the effects of an endogenous cannabinoid ligand, 2-arachidonoylglycerol (2-AG), on the reinforcing properties of exercise reinforcement in lean and obese Zucker rats. Ten obese and 10 lean Zucker rats pressed a locked door under a progressive ratio schedule of reinforcement that, when unlocked, provided access to a running wheel for 2-min periods. After baseline breakpoints were established, doses of 2-AG (0.3-3 mg/kg) were administered before experimental sessions. Obese rats exhibited lower breakpoints for wheel activity, lower response rates, and fewer revolutions compared with lean rats. 2-AG decreased breakpoints, response rates, and revolutions for obese rats, and revolutions only for lean rats. These data suggest that 2-AG may reduce the reinforcing properties of activity, and that obese Zuckers may show a greater sensitivity to 2-AG. The data also suggest that endocannabinoids may play a role in the reinforcing properties of exercise.