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ABSTRACT. Aperia, A., Broherger, O., Thodenius, K. and Zetterström, R. (Department of Paediatrics, Karolinska Institutet, S:t Göran's Children's Hospital, Stockholm, Sweden). Developmental study of the renal response to an oral salt load in preterm infants. Acta Paediatr Scand 63: 517, 1974.—An evaluation of sodium homeostasis in 44 preterm infants with gestational ages between 29 and 37 weeks has been carried out during the first week after birth and until time of expected term. The natriuretic response to an oral sodium load has been studied in all infants and the GFR (single injection technique of inulin) in 17 infants. The results are compared with those previously found in full‐term infants. The natriuretic response was highest and the GFR was lowest in the very preterm neonates. In the very preterm infants the values for sodium excretion and GFR was just about the same at the time of expected term as in full‐term newborns. Various explanations for the difference between the very preterm neonates and full‐term neonates are discussed. One factor of importance might he the anatomical development. The immature kidney has in comparison to the adult kidney relatively larger glomerular than tubular mass. Extra‐uterine life seems to have little influence on the development of GFR as well as on the development of the response to the oral salt load. Thus in the very preterm infants, the postmenstrual rather than the postnatal age should be considered when prescribing fluid, electrolytes and drugs.

Aperia, A., Broberger, O., Herin, P., Thodenius, K. and Zetterström, R. (Department of Paediatrics, Karolinska Institute, St. Göran's Children's Hospital, Stockholm, Sweden). Postnatal control of water and electrolyte homeostasis in pre‐term and full‐term infants. Acta Paediatr Scand, Suppl. 305: 61–65, 1983.—A review is given of the progress which has been made during the last decade within the field of renal control of water and sodium homeostasis in newborn infants of varying gestational age. Both preterm and full‐term infants have a low capacity for rapid excretion of a salt load. The natriuretic response improves gradually up to the age of 15 months. The capacity to excrete a load of sodium bicarbonate is higher than to excrete a load of sodium chloride. Under basal conditions preterm infants of a gestational age below 35 weeks have a higher renal sodium excretion than full‐term infants. They also appear to be unable to retain sodium when in negative balance. The capacity to concentrate the urine is low in newborn infants, the maximal osmolality being only slightly above that of plasma. The concentrating capacity increases relatively fast during the first 4–6 postnatal weeks in full‐term as well as in pre‐term infants but does not reach the adult level until the second year. Water loaded newborn infants are able to excrete a urine with a osmolality as low as 30–50 mOsm per kg. In full‐term infants free water clearance per unit filtered water is higher than in adults. Water‐loaded pre‐term infants with a gestational age of more than 30 weeks also have a supernormal diluting capacity.