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The syndrome of hereditary diabetes insipidus of hypothalamic origin has been described in adult rats.' The task of this paper is to ascertain whether this syndrome is also present in young animals and at what age it begins to be manifest. Inasmuch as this type of diabetes insipidus is caused by the defect of vasopressin synthesis, it is obvious that its symptoms cannot be disclosed before the age at which the presence of vasopressin is detected in normal rats.Neurons of the vasopressin-synthesizing hypothalamic nuclei (i.e., n. supraopticus and n. paraventricularis) are found between days 13-17 of fetal life in rats2, Radioimmunoassayable vasopressin was observed in the rat brain at day 14 and in the pituitary at day 16 of fetal life.6 In his pioneer study from 1947 Hans Heller estimated the vasopressin content in the neurohypophysis to be less than 3 mU and more than 1.5 mU/gland in 1-day-old rats. This is in perfect accordance with recent data obtained using radioimmun~assay.~The osmoregulatory role of neurohypophysis is effectuated mainly by its influence on kidney function. It is mediated by the blood transfer of vasopressin. However, no antidiuretic activity was found in the blood plasma of normally hydrated rats until the age of four weeks.8 Antidiuretic activity corresponding to 4-5 pU/ml was observed in plasma of 23-day-old rats after water deprivation for six hours. Very low but detectable (about 1 pU/ml) activity appeared in plasma of 10-day-old rats loaded by a 3% salt sol~tion.~ From these data it could be predicted that the first signs of impaired concentrating ability of the kidney, accompanied by polyuria and polydipsia, ought to appear in homozygous Brattleboro rats in the course of the second to fourth postnatal week.Young Brattleboro rats were exposed to water deprivation for six hours at the ages of 10, 14, 18, and 22 days. Urine was obtained by perineal stimulation.10-and 14-day-old animals were kept without mother at 30" C, the older ones at 25" C. Urine osmolarity was determined by means of the Knauer osmometer. As shown in FIGURE 1, values of urinary osmolarity in homoand heterozygotes did not differ in 10-and 14-day-old rats whereas two distinct populations were found in 18-and 22-day-old animals. It was assumed that the group with lower osmolarity consisted of insipidic animals while the group with higher urinary osmolarity were animals without diabetes insipidus. This assumption was confirmed by measurement of water intake carried out at the age of 27 days. Water intake did not exceed 25 ml-100 g l -2 4 h-l in individuals with high urine osmolarity and it exceeded 50% of the body weight in animals with low urinary osmolarity.lo Hence, the difference in concentrating

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