In the previous paper 1 hypernatremia in rats was found to be associated with an increase in red cell osmotic fragility. This report concerns the variations of osmotic fragility in infants having hypernatremia and others having hyponatremia. From these studies it has been learned that hypernatremia in infancy is associated with an increase in red cell osmotic fragility and, conversely, hyponatremia is associated with a decrease in red cell osmotic fragility. The osmotic fragility remains abnormal for 1-4 days after the concentration of sodium in serum returns to normal. These are not the expected changes that would occur with changes in sodium chloride concentrations in
The red cell is, at all times, in osmotic equilibrium with its environment, i.e., serum,2 such that a change in the osmolality of the outside environment results in an equal change within the red cell. In instances where changes in osmolality are induced by