An investigation was undertaken of the changes in metabolic energy balance which occur in weightlessness. Daily energy intake was determined each day throughout the 28-, 59-, and 84-day flights for each of the nine Skylab astronauts. The energy content of the urine and feces was also measured. Changes in body composition were inferred from measurements of weight, volume, water, and total exchangeable potassium before and after flight. During flight changes were followed by a daily measurement of body mass and by metabolic balance. Examination of the data reveal losses in body weight during the 1st and 2nd mo of flight, a loss in body water and protein during the 1st mo, and a loss of fat during the 1st, 2nd, and 3rd mo of flight. The energy input was about 41.7 kcal/kg per day on the ground, and 43.7 kcal/kg per day after 3 mo in space. The increase in net energy input of about 1.6% per mo is significant (P less than 0.05). When the net energy input is expressed on the basis of total body potassium, the increase in the resulting "noramlized" net energy input of about 3.7% per mo is also significant (P less than 0.05).