The effect of moderate physical activity (90 min in fasting and 90 min in feeding at 45-50% of VO2max) upon 24h macronutrient utilization was studied in 8 healthy men, after a diet-exercise-adjustment period of 6 days when the subjects were fed Ig protein· kg^-1· d^-1. 9% of the energy intake came from protein, 39% from fat and 52% from carbohydrate. Energy turnover (ET) was determined by indirect and direct (suit) calorimetry. Protein (PROT) metabolism was assessed by a 24h primed, continuous iv infusion of [1-¹³C]leu and [¹⁵N¹⁵N]urea. Total ET was 15.1±1.2 MJ· d^-1 (Mean±SD), resul(ing in a slight positive energy balance of 0.83±0.84 MJ- dj. PROT contributed to 10%, fat 32% and CHO 58% of total 24h ET. During physical activity, the contribution from PROT decreased from 10 to 3% during fasting and from 19 to 5% during feeding while CHO contributed to 59% (fasting), 81% (feeding). Fat contributed to 61% of ET after exercise during fasting. During feeding the percent energy from PROT and CHO increased while that from fat decreased. Subjects with a low fat/high CHO oxidation showed the same pattern throughout the day despite exercise or feeding. Our data on the partitioning of macronutrients for energy, among individuals and during exercise, will be discussed in reference to body composition maintenance.