Clearances of inulin and para-aminohippuric acid were performed in 22 newborn infants without renal disease. Ages varied from 12 hours to 25 days. The mean values of inulin and PAH clearances during the first four days of life were 10.8 +/- 1.0 ml/min/m2 and 30.8 +/- ml/min/m2, respectively. A twofold increase in glomerular filtration rate was observed during the first 14 days of life. A similar increase in PAH clearance occurred during this period. No correlation was found between gestational age (range: 32 to 39 weeks) and GFR in ten neonates studied during the first four days after birth.