Twenty four hour electrocardiograms in 20 patients with Bartter's syndrome, a disorder associated with chronic potassium deficiency, were analysed for atrial and ventricular extrasystoles, pauses (RR interval greater than 2 s), and heart rate. The 12 lead resting electrocardiogram was also evaluated. There were slight electrocardiographic changes with ST segment depression (greater than or equal to - 0.5 mm) in seven patients, flat or low amplitude T waves in seven, and U waves (greater than or equal to + 1.0 mm) in three patients. The QT interval was prolonged in 18 patients. Nine patients had one or more ventricular extrasystoles in 24 hours. Only two patients had more than 200 ventricular extrasystoles in 24 hours. No patient had ventricular tachycardia. A total of nine patients had one or more atrial extrasystoles in 24 hours, but only one patient had more than 200 in 24 hours. One patient had an attack of non-sustained supraventricular tachycardia. No patient had pauses. Dangerous tachycardia was rare in these patients with chronic potassium deficiency caused by Bartter's syndrome. The general pattern of slight electrocardiographic changes may reflect an adaptation of the myocardium to hypokalaemia. Further studies are, however, needed to determine whether these findings are relevant to long term prognosis.