Oxidative and nonoxidative macronutrient disposal rates were measured in lean and obese males randomly fed mixed meals containing 0, 33, 67, 100, and 134 kJ/kg fat-free mass (0, 8, 16, 24, and 32 kcal/kg). Body composition, preprandial and postprandial energy expenditure, and macronutrient concentrations in the extracellular space were measured. Relationships among carbohydrate, fat, and protein disposal rates; body weight; and body composition were examined. Oxidative and nonoxidative disposals of macronutrients were not different between the lean and obese groups. Glucose was preferentially oxidized and fat was preferentially stored after nutrient ingestion. Macronutrient storage increased linearly with caloric intake. Oxidative and nonoxidative macronutrient disposals were completed within 8 h after ingesting the meals. Serum insulin concentrations rose to 3000-6000 pmol/L in two obese men after their two largest meals. Eight hours after nutrient ingestion, concentrations of macronutrient substrates, metabolic products, and insulin were indistinguishable from preprandial values.