It is generally agreed that the maintenance requirements for water of individuals is determined by their caloric expenditure. By means of the following formulae, the caloric expenditure of hospitalized patients can be determined from weight alone. For weights ranging from 0 to 10 kg, the caloric expenditure is 100 cal/kg/day; from 10 to 20 kg the caloric expenditure is 1000 cal plus 50 cal/kg for each kilogram of body weight more than 10; over 20 kg the caloric expenditure is 1500 cal plus 20 cal/kg for each kilogram more than 20. Maintenance requirements for water depend upon insensible loss of water and renal loss. An allowance of 50 ml/100 cal/day will replace insensible loss of water, and 66.7 ml/100 cal/day will replace the average renal loss so that the total requirement is 116.7 ml/100 cal/day. As water of oxidation will supply approximately 16.7 ml/100 cal/day, the remaining 100 ml/100 cal/day must be supplied to meet the remaining water losses of patients on parenteral fluid therapy. Possible exceptions to this figure are discussed. Maintenance requirements of sodium, chloride and potassium are 3.0, 2.0 and 2.0 mEq/100 cal/day, respectively.