The purpose of this study was to examine the possible relationship between the thermogenic response to a mixed meal and the aerobic capacity in healthy subjects. Fourteen male subjects participated, and their maximal oxygen uptake was determined on a bicycle exercise ergometer. Two groups, each comprising seven individuals, were compared: a well-trained group, with an oxygen uptake of 58 +/- 2 ml min-1 kg-1 and a sedentary group, with an oxygen uptake of 39 +/- 2 ml min-1 kg-1. Respiratory gas exchange was measured continuously for 1 h in the basal state and then for 3 h postprandially. The subjects ingested a test meal in liquid form, consisting of 17% kJ protein, 28% kJ lipids and 55% kJ carbohydrates, and corresponding to 60% of the individually computed 24-h basal energy expenditure. Basal oxygen uptake and energy expenditure were similar in the two groups. After the meal, pulmonary oxygen uptake and energy expenditure rose rapidly and reached a plateau after 1 h. The responses were no different in the two groups: the average rise in pulmonary oxygen uptake above basal during the whole study period was 24.0 +/- 2.1% in well-trained and 26.7 +/- 1.5% in sedentary subjects (NS); the corresponding values for energy expenditure were 25.0 +/- 2.1% and 29.0 +/- 1.6% (NS). Also, when expressed in absolute terms the increments above basal were not significantly different. There was no discernible relationship between the individual thermogenic response and maximal oxygen uptake. In conclusion, the present findings do not indicate that diet-induced thermogenesis is proportional to aerobic capacity in healthy young men.