PERFUSION with potassium deficient solutions induces ectopic rhythms and atrioventricular (AV) conduction disturbances in isolated rabbit hearts.1,2In dogs and in man, potassium loss sensitizes the heart to digitalis intoxication which is manifested by ectopic beats, ectopic rhythms and AV conduction disturbances.3Levine4suggested that potassium loss may precipitate various arrhythmias, particularly supraventricular tachycardia, in patients receiving and in patients not receiving digitalis. Recently Fisch and co-workers5have reviewed many case reports of patients in whom ectopic beats and AV conduction disturbances were attributed to potassium deficiency or hypopotassemia. Surawicz and Lepeschkin6studied electrocardiograms of 25 patients with hypopotassemia and found that all four patients with plasma potassium concentration less than 2.6 mEq/liter had ectopic beats. In another study,713 of 50 patients with plasma potassium concentration less than 3.5 mEq/liter had ectopic beats. Weaver and Burchell8found ectopic atrial beats in