In dogs anesthetized with pentobarbital, ventricular arrhythmias induced by ouabain were consistently reverted to sinus rhythm by acute lowering of the serum calcium. In ouabain-in-toxicated dogs, the depressed and shortened S-T segment was elevated and lengthened by induced hypocalcemia. Potassium administered concomitantly with ouabain exerted a protective effect against ouabain intoxication. Lowering of the serum calcium in animals showing moderately advanced potassium effects in their ECG precipitated acute, fatal potassium poisoning. Ouabain was capable of reversing the tachycardia and pulsus alternans due to hypocalcemia in the face of persistent hypocalcemic tetany.