Potassium has been rapidly removed from the circulating blood of dogs in 22 experiments in which other extracellular electrolytes were maintained constant. In these experiments potassium extraction occurred in an acute phase accompanied by a reduction in serum level and a second phase of continued extraction with little change in the extracellular concentration. In the first phase the P wave increased markedly in amplitude and width. The A-V conduction time became prolonged. The QRS complex widened, the T wave became broadened and rounded, and the S-T segment depressed, the latter change occurring in the second phase of dialysis. In these experiments, acute depletion of potassium in dogs consistently reflects the electrocardiographic changes ascribed to hypokalemia.