Glomerular filtration rate (GFR) was measured in 39 healthy infants (gestation 27-40 weeks, birthweight 0.68-3.71 kg) by prolonged inulin infusion between 2 and 63 days of age. Absolute GFR showed a logarithmic rise with conceptional age (gestational plus postnatal age) which was independent of postnatal age. GFR per kilogram showed a slow rise with gestational age, and a rapid rise with increasing postnatal age which was shown to be due to a temporary cessation of growth rather than a true acceleration in renal maturation. GFR per unit surface area showed similar, but steeper rises. Formulae were constructed to predict GFR in the first month of life from postnatal age (PA), weight and birthweight (BW); (GFR = (0.24 BW + 0.18 PA + 0.45) X weight), or from conceptional age (CA); (GFR = 100.0618 CA-1.859). 95% of the predictions fell within 66 and 151%, and 58 and 172% of the measured values, respectively. Data from 14 studies were expressed in the same format where possible. The agreement between the reported data and this study was close. Apparent contradictions between these studies had been largely due to their different forms of presentation.