Undereating for 6 weeks by 1000 kcal/d (4186 kJ/d) was studied in four women and five men following a 3-week control period when food intake was adjusted to expenditure. Sedentary energy expenditure was measured by direct and indirect calorimetry, food and waste by bomb calorimetry, and loss of body stores was calculated from body density and total body water. All nine subjects lost weight fairly steadily for the 6 weeks, reducing fat stores by 4.4 kg and body water by 1.0 kg on average. Sedentary energy expenditure declined by 12 per cent; energy losses in body waste decreased a little. Energy balance calculations from the quantities measured accounted for all but 362 kcal/d (1515 kJ/d) during undereating; the imbalance was thought to be the result of a smaller reduction in the expenditures of free activity, which was not measured, than in the measured reduction of sedentary expenditure.