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Indices of renal excretion and reabsorption of phosphate were studied in 20 neonatal infants, 20 infants aged 3 months, and 20 infants aged 6 months. All subjects were normal and were fed a modified formula enriched with vitamin D. In neonatal infants all indices of phosphate excretion were found to be significantly lower and those of phosphate reabsorption significantly higher than in older infants. Phosphate excretion gradually increased with age, while its reabsorption decreased. The positive correlation between serum phosphorus and renal threshold phosphate concentration (TmP/GFR) and the negative correlation between phosphorus excretion index and TmP/GFR found in this study shows that in young infants as in adults TmP/GFR is the principal determinant of renal phosphate homeostasis. Among the many indices of renal phosphate handling in use TmP/GFR is the best for studies of phosphorus or calcium metabolism disorders, or both, especially in the first three months of life. [ABSTRACT FROM AUTHOR]