The sodium, potassium and nitrogen balances of 24 small premature infants during the first 3 weeks of life have been determined. During the first week of life there was a sodium deficit of 6 to 16 meq/kg in clinically well infants. Prior to milk feeding there was a loss of potassium and nitrogen, followed by change to retention once milk feedings were started.

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Some sick infants showed a similar pattern; others showed a relative salt retention during illness or during moderate increases in sodium intake.

Three infants were treated with cortisone during the first week of life. Two retained potassium as well as sodium during that time. Another demonstrated an apparent migration of sodium into the cells while receiving the cortisone.

Low and more labile sodium values in serum were noted more often in the ill infants, while higher potassium levels in serum were more frequently encountered in the same infants, although these two phenomena were not necessarily associated.

During the second and third weeks of life, all babies studied began to retain sodium irrespective of the degree of immaturity at birth and the positive nitrogen and potassium balance continued.

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It is conjectured that the sodium diuresis observed during the first week of life may represent a physiologic reduction of the total body water.

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