Water contents of the various body compartments were estimated before and after a 7- to 10-h period of extracorporeal membrane oxygenation in five healthy baboon neonates. Total body water, extracellular water, and plasma volume were estimated simultaneously by antipyrine, bromide, and T-1824 dilution. Volumes of intracellular water, interstitial water, and blood and red cells were calculated from the experimental estimates. Mean preextracorporeal membrane oxygenation estimates of body water volume were in agreement with those previously reported in baboon neonates. During extracorporeal membrane oxygenation, no statistically significant changes occurred in the water content of the various body compartments. This absence of changes was contrasted to the changes demonstrated in human infants and adults undergoing intracardiac surgery with extracorporeal oxygenation and hypothermia and various hypotheses were put forward to explain the different changes observed.