Ten male albino rats paired on the basis of running activity in an activity wheel were than placed each in a braked activity wheel equipped with a Skinner-type bar. When the exptl. animal of the pair pressed the bar, both it and its control partner (in another wheel) were permitted to run for 30 sec. When the control animal pressed the bar the brakes were not released and the rats could not run. After a schedule of regular and periodic reinforcements the exptl. rats were pressing the bar significantly more than the control animals. It was concluded that the opportunity to run in an activity wheel is an adequate reinforcement for the instrumental response of bar pressing.