The rapid initial phase of intravenous rehydration was studied in 51 patients with diarrheal dehydration. All received variable amounts of potassium, in addition to the standard mixture of equal parts of 5% glucose and 0.9% sodium chloride, at the rate of 50 ml/kg/hours. Final concentration of potassium in the infusion fluid ranged from 4 to 21,3 meq/l, achieving an infusion velocity that ranged from 0.08 to 1.38 meq/kg/hour. No untoward effects were observed with the use of potassium; the average serum potassium levels at the end of the period did not increase above those observed on admission (4,2±1 vs 4±0.9 mcq/l). A subgroup of patients underwent metabolic balances. In 8 patients no potassium was administered and an intracellular deficit (as estimated with the “chloride space” of Darrow) of 0,48±0,36 meq/kg, was observed. In another group of 11 patients, who received potassium, intracellular gain of 0,23±0,38 meq/kg was observed. These results point to the fact that potassium can be used safely, even before diuresis ensues, in patients with diarrheal dehydration that need intravenous therapy.