This experiment tested the effects of reinforcing obese children lo be more active or less sedentary in their choice of active versus sedentary behaviors. On days I and 5, there were no contingencies for sedentary or active behaviors. During days 2 through 4, children in the Activity group were reinforced for being more active, and they significantly increased their activity and decreased time spent on preferred sedentary activities. Children in the Sedentary group were reinforced for not engaging in preferred sedentary behaviors, and they significantly decreased time spent on these sedentary behaviors, with lime reallocated both to being more active and to substitution of lower preference sedentary behaviors. Children randomized to the Control group were reinforced for attendance and made choices among the alternatives as usual, allocating most or their time during all 5 days for their preferred sedentary behaviors. These laboratory results support the idea that activity can be increased by either reinforcing children for being more active or for reducing lime spent in sedentary activities.