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The effects of hypoxia and feeding on red cell inorganic phosphate (Pi) concentrations were studied in neonates. Although hypoxia caused a rise in extracellular Pi, the intracellular concentration of this ion did not change in comparison to control infants of the same age (first 24 h). As a result of these changes, the distribution of phosphate ions across the erythrocyte membrane was significantly lower in the hypoxic infants than in the controls. In the hypoxic infants, adenosine triphosphate (ATP) levels in the red cells were found significantly lower than in controls, while the 2,3-diphosphoglycerate (2,3-DPG) levels were raised. In breast-fed 2 or 3-day-old neonates, both plasma and red cell Pi were found to be increased but to different degrees, affecting therefore the molar Pi distribution, which was lower than in the controls of the first day. In these infants, ATP was lower and 2,3-DPG higher than in the controls of the first day of life. These findings suggest that intracellular phosphate metabolism in neonates does not follow extracellular phosphate changes. Further investigation is needed to elucidate the controlling factors.