The concentrating ability of the kidney changes with age: after birth it increases (Smith, 1959), and in old age it falls (Lindeman, van Buren, and Raisz, 1960). The maximum osmolality during the new-born period is much lower than in adults (Heller, 1944; Smith, Yudkin, Young, Minkowski, and Cushman, 1949; Barnett and Vesterdal, 1953; Hansen and Smith, 1953; McCance and Widdowson, 1954; Calgagno and Rubin, 1960; Fisher, Pyle, Porter, Beard, and Panos, 1963). This smaller concentrating ability rapidly increases after birth, as shown by the significantly higher maximum osmolality of the urine in mature and some premature children in the second to fourth week of life (Pratt and Snyderman, 1953; Edelmann, Barnett, and Troupkou, 1960), and may reach 1,473 mOsm./l. at the age of 35 days (Pratt, Bienvenu, and Whyte, 1948), and 1,570 mOsm./l. at 73 days (Drescher, Barnett, and Troupkou, 1962). The osmolality of the urine of premature infants in the first month of life can be low (Calgagno, Rubin, and Weintraub, 1954), particularly in premature infants with a birth weight below 2,250 g. in comparison with that of mature infants of the same age (Vocel, Polacek, Neugebaurova, and Sebkova', 1963). Results in a number of series of children investigated with various tests showed a tendency for maximum osmolality to increase after the second month of life (Winberg, 1959; Polacek and Polanska', 1962; Martinek, Janovsky, and Stanincova, 1962) and even after the first year of life (Winberg, 1959; Polacek, 1962). The comparison of these data is difficult because the methods used to achieve increasing concentration of urine were diverse. For this reason it seemed desirable to investigate subjectsfrom the early postnatal period up to adult life, using a single method. Since premature infants with a low birth weight react to fluid restriction with lower concen-tration of urine, and sometimes even by haemocon-centration and raised non-protein nitrogen (Vocel et al., 1963; Fisher et al., 1963), only those premature infants with a birth weight over 2,250 g. were included in the series.