Total body water (TBW), extracellular volume (ECV) and intracellular water (ICW) were measured in a cross-sectional study of 107 infants up to four weeks after birth. Three groups of infants were selected for study: (1) mature normally grown infants, (2) mature low weight for gestational age (LWGA) infants and (3) premature normally grown infants. In the normal mature infants there was no significant change in TBW during the first 6 days after birth but there was a small but significant (P less than 0.02) redistribution of extracellular water into the cells by the sixth postnatal day. This suggests that the normal weight loss in infants after birth is due to a relative starvation rather than cell dehydration. In the LWGA infants, TBW levels were higher than normal and ICW significantly increased. This index of cell mass further increased throughout the 14-day period studied (P less than 0.01) and was the highest of all groups studied. It is argued that the changes are due to cytoplasmic growth. Premature babies (mean weight approximately 2000 g and greater than 30 weeks gestation) had higher TBW values than their mature normally grown counterparts. Hyponatraemia was infrequent and no shift of water into cells was detected. All groups of infants revealed loss of ECV over the first two weeks and in premature infants the loss was commensurate with that of TBW.