The clinical response and changes in potassium balance were studied during oral rehydration therapy (ORT) with a solution containing 20 mmol potassium per litre in well-nourished and malnourished infants of 3 to 15 months of age as well as in neonates and young infants less than 2 months old (range 5-60 days). All infants were successfully rehydrated orally. The potassium intake considerably exceeded the potassium output in all groups during the entire rehydration period. The stool and urine potassium output were about the same in the 3 groups of patients. The retention of potassium increased during ORT in all groups at about the same rate. The study thus demonstrates that an oral rehydration solution (ORS) containing 20 mmol potassium per litre provides enough potassium to all patients. An increase in the potassium concentration of the ORT is therefore not recommended.