Rats were required to press a bar to activate a motor-driven wheel that forced them to run and subsequently to drink to turn off the wheel. Barpressing and lickilng increased, showing the onset and offset of running to be positively and negatively reinforcing, respectively. The experimental control of the offset of running, in contrast to the traditional control for onset only, served to demonstrate that since organisms stop such behaviors as they start, self-initiated behaviors will act as negative as well as positive reinforcers.