In 51 infants (0.5-12 months) and 143 children (1-15 years), the postnatal development of renal phosphate handling could be studied by short term clearance investigations. The infants demonstrated significantly higher values of plasma phosphate (Pp), urinary phosphate excretion and endogenous phosphate clearance than the children. Net tubular phosphate reabsorption (Tp) was low infancy due to low glomerular filtration rate (CIn). The fractional phosphate reabsorption (Tp/CIn), however, was significantly higher in infancy than in childhood. There was a close correlation between fractional phosphate reabsorption and plasma phosphate for both children and infants. When the regression lines of Pp to Tp/CIn were analyzed separately for children and infants, a parallel shift was recognized, which means that at each level of Tp/CIn infants had higher plasma phosphate concentrations than children. Evaluation of available data suggests that the shift was very probably related to low CIn in the young infants which may lead to further retention of phosphate.