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It is known that under comparable conditions of dehydration newborn and infant animals are unable to form urine of as high an osmotic concentration as adult animals [S, 171. The difference in concentration capacity in the young and in the adult is more pronounced the less mature the newborn. This low concentration capacity has also been described for human infants, and it has been stated that the mean maximum limits of osmolarity in children in the first weeks after birth are about 800 mOsm/L, which means that they only slightly exceed half the level of urine osmolarity in adults [9, 181. It is believed that functional and structural immaturity of the hypothalamo-neurohypophyseal system producing antidiuretic hormone (ADH), and insufficient sensitivity of the distal portions of the nephron to ADH are involved. The level of antidiuretic activity (ADA) in the neurohypophyses of newborn rats and puppies is very low [ 2 , 71. Even in newborn children the ADA content in the neurohypophysis does not reach the level of adults [lo]. In addition, thc findings of Rodeck [25, 261 ~~~ .. Technic.<:l ar,lstnnc~ V. 13dlmv& and H. Boui.kova. indicate undeveloped neurosecretion in the hypothalamus of newborn children. Under normal hydration ADA cannot bc identified in the plasma of infant rat before the 23rd day of life [3], and in puppies not before the eleventh day of life [2]. Under dehydration or after a NaCl load ADA can be detected in the plasma already in 10day-old rats and 5-day-old puppies [a, 41.Heller & Hradcovj [ l l ] have found ADA in the plasma of normally hydrated children only a t the beginning of the 5th month.In the present work it was enquired whether, after an osmolar stimulus, ADA can be detected in the plasma of even younger infants. Part of the findings has been published as a preliminary communication [14, 201. MethodsDisposal of an NaCl load NaCl (0.7 g/kg body weight dissolved in two thirds of the child's normal milk intake) was administered to 50 mat,ure children between thc ages of 1 6 clays and 6 months. The NaCl dosc was chosen in siich a ~v a y as not t o allow the relative wat.cr tieficit. rcsulting from administration of D hyp:rt.onic solution into the gastrointestirinl tract, t o excet:d 5 % of t,he body woight. Clost. be-

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