This laboratory study examined whether making sedentary activities contingent upon being physically active would increase obese children's physical activity. Fourteen obese children aged 8-12 y participated in a baseline session in which they had free choice among a variety of sedentary activities and riding a stationary bicycle. Children were then randomized to either a contingent group in which watching video cassette recorder (VCR) movies and playing video games were contingent upon riding the bicycle or a control group in which all physical and sedentary activities remained freely available. Contingent group children increased physical activity and decreased television activities in comparison to the control, even though other sedentary activities remained freely available. Findings suggest that highly valued sedentary activities can reinforce physical activities and that sedentary activities do not completely substitute amongst themselves. The automated system used to make television activities contingent upon physical activity has potential for modifying activity in the treatment of obesity.