The effects of two diets equal in protein content (12% of energy) but different in carbohydrates (80% of energy) or in fat (68% of energy) were studied in a changeover trial with 6 healthy young subjects. Each person was 2 weeks on each diet. Under the conditions of slight undernutrition and neutral temperature the balances of nitrogen, carbon and energy were assessed in 7-day collection periods, and according to 48-hour measurements of gaseous exchange (carbon-nitrogen balance method) by the procedures of indirect calorimetry. At isocaloric supply of metabolizable energy no differences were found in the retention of N, C and energy. The heat production calculated by indirect calorimetry was not significantly different with both diets. The lower body weight during the high-fat diet was due to higher sodium and water excretion. The high-fat diet resulted in lower fecal and higher urine excretion of nitrogen, but because of a compensative response no difference in nitrogen retention was observed between the two dietary regimens. The plasma concentrations of cholesterol, glucose and uric acid were higher with fat-rich diet; those of triglycerides were higher with carbohydrate-rich diet. The results demonstrate that the composition of the diet did not influence the energy balance.