Renal tubular function has been studied in pig fetuses of 105-112 d gestational age in new-born pigs 5-9 d old. The experiments were performed on anaesthetized animals, urines being collected by inserting a catheter into one ureter of the animal under study. The glomerular filtration rate was estimated and plasma concentrations and urinary excretion of the following substances were measured: sodium, potassium, calcium, ammonia, urea, phosphate, glucose fructose, creatinine, protein and exogenous 4-aminohippuric acid, and inulin. The reabsorption of water was considered in relation to the plasma vasopressin values. New-born pigs were loaded with glucose and fructose in order to determine the maximal tubular transport rate of these substances. Significant changes at birth occur in only a few functions of the tubulus system. Following delivery, major changes are: (1) the increased reabsorption of sodium and water which is probably the most important adaptation to extra-uterine life; (2) an apparent increasing impermeability of the tubular epithelium for creatinine, and (3) the direction of transport of fructose, which is reabsorbed by fetuses whereas neonates demonstrate a net secretion. Glucose and fructose are transported by different mechanisms. The experiments with fructose-loaded piglets demonstrate that there are at least two transport mechanisms for fructose: reabsorption - either passive or active - and secretion. The factors causing a shifting from one mechanism to the other are not yet known.