The thermogenic responses of ten lean and ten moderately obese women to food, exercise, and food plus exercise were measured using open circuit respirometry for five minutes every half hour for four hours under six conditions: during five minutes of bicycle exercise at a workload of 300 kpm/min with and without eating a 910 kcal mixed meal; cycling at a workload just below the anaerobic threshold with and without food; and at rest with and without food. Over the four-hour period, the thermic effect of food at rest was similar for the lean and obese groups: 50 kcal and 47 kcal, respectively. Eating before exercise increased the exercise metabolic rate by 11% for the lean women and by 4% for the obese women (P less than 0.005). Exercise potentiated the thermic effect of food for the lean women but nor for the obese women: the thermic effect of food was 2.54 times greater during exercise than at rest for the lean group, but only 1.01 times greater for the obese women (P less than 0.005). This reduced response to the combined stimulus of food plus exercise may constitute a subtle metabolic factor associated with obesity.