SURELY the pediatrician who looks over a group of newborn infants is increasingly aware of their heterogeneity, recently emphasized by Sinclair in a commentary in Pediatrics. Those of like size may be dissimilar in gestational age; the distribution of water and electrolytes differ on day 3 of postnatal life from day 1, after the obligatory loss of water and electrolytes; and body composition changes markedly during gestation. Widdowson, in a summary of many of the compositional changes in fetal life noted that at 1 kg weight the body is 86 to 90% water, at 2 kg it is 72 to 80% water, and at 3 kg it is nearer 75% water.