The sodium and water intakes of a group of infants treated for diarrhea and dehydration in 1972-73, when hypernatremia was common, and a group of infants treated for diarrhea and dehydration in 1978-79, when hypernatremia was rare, were compared. Infants from both study periods whose intake was poor and who received feedings containing sodium in concentrations greater than 17 mEq/liter were more likely to develop elevated serum sodium con centrations. In a group of infants recovering from diarrhea, the effect upon the serum concentration of sodium of varying water and sodium intake was studied. Those treated with small amounts of a solution containing 30 mEq sodium per liter regularly demonstrated an elevation of serum sodium, whereas those treated with large amounts of the same solution regularly demonstrated a depression. It is concluded that infants with diarrhea should receive feed ings containing sodium in concentrations less than 17 mEq/liter unless a high fluid intake can be assured.