Sixty clinically well infants of various gestational ages (27 to 40 weeks) were studied from 24-40 hours after birth to evaluate glomerular filtration rate and renal excretion rate of sodium at various stages of fetal maturation. Creatinine clearance was directly related to gestational as (r = 0.643). Fractional sodium excretion was inversely related to gestational age (r = -0.755). The renal functions of small for gestational age infants were similar to those of full-term infants whose birth weights were appropriate for gestational age. The data showed that the glomerular functions of an infant below 32 weeks of gestation were more predominant than the tubular function resulting in a greater fractional sodium excretion rate and higher urinary Na loss in infants of this gestational age, when compared with the more mature infants.