Antipyrine space (APS) studies in 44 normally grown (NG) neonates revealed estimates of total body water (TBW) comparable to those found by earlier investigators utilizing desiccation analyses. Body water per kilogram of body weight was higher in the 32 premature infants than in the 12 mature neonates, even after correction for presumed changes in body fat during growth (mean APS/lean mass was 846 ml/kg in premature and 782 ml/kg in mature infants,P< 0.01), suggesting less hydration of lean mass with maturity. Cell water (ICW) estimates, obtained by concurrent bromide and antipyrine dilution studies revealed no differences between mature and premature neonates after fat corrections (mean ICW/lcan mass was 391 ml/kg in premature and 368 ml/kg in mature neonates,P> 0.10). Prompt decrease in cell water was evident in the mature NG infants (correlation coefficient for ICW/APSversusstudy age:r= −0.76; ICWversusstudy age:r= −0.58), whereas such changes were absent in the premature infants (ICW/APSversusstudy age:r= −0.08; ICWversusstudy age:r= −0.16).Total water and cell water estimates in 23 intrauterine growth-retarded (IGR) neonates also studied were comparable to those of weight peers (mean APS, 790 ml/kg and ICW, 379 ml/kg in IGR; mean APS, 809 ml/kg and ICW, 375 ml/kg in NG prematures) but considerably in excess of values seen in gestational peers (mean APS, 688 ml/kg and ICW, 324 ml/kg in NG mature infants). The high total body water and cell water values were particularly prominent in the earlier-studied IGR infants (mean APS, 841 ml/kg and ICW, 494 ml/kg); a notable finding was the rapid downward adjustment of cell water to values similar to those in NG mature infants (ICW, 335 ml/kg in later-studied IGR infants and 314 ml/kg in later-studied NG mature neonates). This expansion of cell water in earlier-studied IGR infants persisted despite correction for a presumed total depletion of body fat (ICW/lean mass, 494 ml/kg in IGR infants, and 389 ml/kg in NG mature babies; ICW/APS was 0.56 in IGR neonates and 0.50 in NG mature infants,P< 0.001 for both). These findings indicate a real increase in cell water at birth in the IGR infant.SpeculationChanges in body composition in infants with intrauterine growth retardation (IGR) include sizable expansions, on a per kilogram basis, of all body water compartments. Although these findings resemble those observed in chronic protein-calorie malnutrition, rapid adjustments toward normal in the early hours after birth suggest a more transient, acute expansion, primarily of cell water, in the IGR neonate. These changes may reflect impaired cellular metabolism and increased cell acidity consequent to an increased asphyxial stress of labor and delivery in these infants