The thermic effects of food and exercise were studied on groups of normally [10] weighted and obese [14] subjects using oxygen consumption as the criterion of energy expenditure. The results indicated that the increased resting metabolism resulting from the ingestion of a 4.200 kJ mixed content meal (thermic effect) was similar in the two groups (12-17%). But the further potentiation of this thermic effect resulting from exercise following a meal was virtually absent in the obese group (0.8%), while in the normally weighted controls it amounted to a further 17%. It is suggested that the absence of exercise-induced potentiation in the obese may help to explain their difficulty in regard to maintenance of "normal" weight and weight reduction.