SummaryA postnatal contraction of extracellular fluid occurs in low birth weight infants. Patterns of postnatal renal maturation were assessed with the assumption that changes in body composition were mediated in part by the developing kidney. Twenty-two appropriate for gestational age, low birth weight infants (birth weight mean = 1380 g, gestational age mean 31 weeks) were studied between 12 hr and 61 days of age to evaluate simultaneously glomerular and tubular functional maturation. Since most low birth weight infants have respiratory morbidities (respiratory distress followed by chronic lung disease), the infants were grouped into:group I(13 infants), transient or absent respiratory morbidities; andgroup II(9 infants), persistent and severe respiratory morbidities. Sodium excretion decreased with postnatal age in both groups. Sodium intake did not vary with postnatal age. The percentage of fractional sodium excretion was inversely related to postnatal age. Creatinine clearance correlated directly with postnatal age in both groups. Increased sodium excretion and percentage of fractional sodium excretion in the first 10 days of life may reflect extracellular fluid solute losses through the kidney. The premature kidney matured in a balanced fashion and persistent respiratory morbidities did not alter this pattern.SpeculationThe kidney of low birth weight infants probably plays an important role in the regulation of body fluid during the first week of life. The adjustment in renal tubular handling of sodium with greater sodium loss in the first days reflects its compensation to fit the demand of greater sodium excretion from the contraction of the extracellular fluid compartment. In an infant with a stable circulatory status, the presence of respiratory complications did not influence this adjustment process.