Total body water was determined in 24 normal infants and children by deuterium oxide dilution and by the antipyrine method. A micro-modification of the antipyrine method is described. Agreement between the two methods for determining total body water was good. Total body water was found to be highest in premature and newborn babies, the values ranging from 70 to 83% of body weight. During the first 6 months of life there was a gradual decrease of body water as per cent body weight. From 6 months to 11 years of age, the values varied between 53 and 63% with no correlation to age or sex.