The development of renal function was studied in neonates with gestational ages ranging from 28 to 43 weeks. The effect of gestational age on the maturation of renal function was assessed in newborn infants studied during the first 72 h of life. Postnatal maturation was examined during the first 3 weeks of life. None of these neonates presented any cardiopulmonary or renal disturbances. Inulin and PAH were used as markers of glomerular filtration rate and effective renal plasma flow, respectively. From the 28th to the 35th week of gestation, there is a progressive increase in inulin and PAH clearances (r = 0.93; p less than 0.001 and r = 0.81; p less than 0.005, respectively), which then level off up to the end of gestation. A linear increase in systemic blood pressure is observed during the last 12 weeks of gestation (r = 0.88; p less than 0.001). There is a positive significant correlation between the rise in blood pressure and inulin or PAH clearance (p less than 0.005). Postnatal maturation of renal function is similar in preterm and term neonates. In both groups birth is followed by a sharp rise in inulin and PAH clearances, a twofold increase of these parameters being observed during the first two weeks of life.