Balances of nitrogen, sodium, chloride, potassium, phosphorus, and calcium are reported during recovery from dehydration due to severe diarrhea. The balances are correlated with each other to bring into view the disturbances in composition of the intracellular as well as the extracellular fluids. Diarrhea led to a decrease in extracellular water owing to loss of sodium andchloride in extracellular fluid. In some cases, a large part of the loss of extracellular sodium was explained by transfer of sodium from the extracellular to the intracellular fluids. Such a transfer of sodium explained the acidosis in one case. In other cases, sodium was lost from the intracellular as well as extracellular fluids. Loss of intracellular potassium was a prominent feature of all cases and was equivalent in two cases to about one-fourth of the estimated normal potassium content of the baby. This loss of potassium is in excess of the loss which would be expected from loss of nitrogen. No great disturbance in the relative concentration of intracellular phosphorus was demonstrated. The usual replacement therapy of fasting and the parenteral administration of solutions containing sodium chloride, sodium lactate, and glucose corrected the extracellular concentrations. However, considerable sodium entered the cells in two cases and loss of potassium was aggrevated. Replacement of intracellular potassium was obtained by adding 1 to 2 Gm. of potassium chloride to submaintenance feeding. One observation indicated that replacement of intracellular potassium is not likely to occur if the diarrhea continues and submaintenance milk feedings without added potassium chloride are given. The data indicate that the usual milk mixtures contain sufficient potassium to replenish the tissues if these mixtures can be taken in the amounts necessary for growth. During the period of fasting, retention of intracellular potassium can beproduced by adding potassium chloride to solutions containing sodium chloride and sodium lactate. Such a treatment restores extracellular concentrations and replaces the deficit of potassium in part. Accompanying this replenishment of both intracellular and extracellular electrolyte, the babies looked better than after the usual type of treatment and recovered from attacks of diarrhea that would usually be fatal. The significance of a deficit of potassium is discussed. Balances of nitrogen, sodium, chloride, potassium, phosphorus, and calcium are reported during recovery from dehydration due to severe diarrhea. The balances are correlated with each other to bring into view the disturbances in composition of the intracellular as well as the extracellular fluids. Diarrhea led to a decrease in extracellular water owing to loss of sodium andchloride in extracellular fluid. In some cases, a large part of the loss of extracellular sodium was explained by transfer of sodium from the extracellular to the intracellular fluids. Such a transfer of sodium explained the acidosis in one case. In other cases, sodium was lost from the intracellular as well as extracellular fluids. Loss of intracellular potassium was a prominent feature of all cases and was equivalent in two cases to about one-fourth of the estimated normal potassium content of the baby. This loss of potassium is in excess of the loss which would be expected from loss of nitrogen. No great disturbance in the relative concentration of intracellular phosphorus was demonstrated. The usual replacement therapy of fasting and the parenteral administration of solutions containing sodium chloride, sodium lactate, and glucose corrected the extracellular concentrations. However, considerable sodium entered the cells in two cases and loss of potassium was aggrevated. Replacement of intracellular potassium was obtained by adding 1 to 2 Gm. of potassium chloride to submaintenance feeding. One observation indicated that replacement of intracellular potassium is not likely to occur if the diarrhea continues and submaintenance milk feedings without added potassium chloride are given. The data indicate that the usual milk mixtures contain sufficient potassium to replenish the tissues if these mixtures can be taken in the amounts necessary for growth. During the period of fasting, retention of intracellular potassium can beproduced by adding potassium chloride to solutions containing sodium chloride and sodium lactate. Such a treatment restores extracellular concentrations and replaces the deficit of potassium in part. Accompanying this replenishment of both intracellular and extracellular electrolyte, the babies looked better than after the usual type of treatment and recovered from attacks of diarrhea that would usually be fatal. The significance of a deficit of potassium is discussed.