Total Body Potassium (TBK) was measured by whole body counting of 40K in 3 patients with Bartter's syndrome before, after 3 months and after 1 year of treatment with enalapril. In 2 patients TBK was found to be decreased before treatment, whereas TBK was within the normal range in the 3rd. During treatment serum potassium concentration and TBK increased in each subject and symptoms of fatigue and tetany disappeared. Enalapril is shown to be an effective treatment in Bartter's syndrome as it improves serum potassium, TBK and complaints.