This study has been designed to examine the effect of gestational age (GA) on the postnatal development of renal function and has been performed in pre-term (PT) infants (GA=30-34 weeks) and in full-term (FT) infants (GA=39-41 weeks). Postnatal age has ranged from 1-35 days. From 8 hour urine samples collected after spontaneous voiding and a capillary blood sample, determinations have been made of the clearance of creatinine (CCr), the fractional excretion of beta 2-microglobulin (FE beta 2) and the fractional excretion of sodium (FENa). In some infants receiving fluid parenterally, simultaneous determinations were made of the clearance of creatinine and inulin. As judged from this study, CCr is a reliable indicator of the glomerular filtration rate (GFR). GFR was almost the same in newborn PT and FT, but from 0.3--1 week of age GFR increased significantly more rapidly in FT than in PT. From 1--5 weeks of age GFR increased at approximately the same rate in PT and FT infants. The absolute value for GFR in 3--5 weeks old infants was lower in PT than in FT. FE beta 2 was higher in PT than in FT infants during the entire first month of life and FENa was higher in PT than in FT infants during the first week of life, suggesting a glomerular tubular imbalance at least at the level of the proximal tubule in PT infants. It is concluded that different stages of maturation will alter the preconditions for the renal adaptation to extrauterine life during at least the first month of life. Therefore special attention must be paid to the limited renal function in PT during their entire first month of life.