Variations in serum potassium (K+) and electrocardiographic changes at different infusion speeds of intravenous K+ were studied in dogs. Solutions of 20, 40, 60 and 80 mEq of K+ were infused in one hour to dogs with normal renal function and normal serum K+. Nine dogs were studied: three without prior hydration and six with previous hydration. Infusing 20 mEq/hour of K+ produced an increase in heart rate without changes in the electrocardiogram. With 40 mEq/hour there were changes in the "t" and "p" waves, and with 60 and 80 mEq/hour, alterations consistent with cardiac ischemia and ventricular tachycardia. Dogs without hydration had more obvious and fatal electrocardiographic changes. Doses of 20 mEq/hour of K + caused no morbidity or mortality in the animals studied. Higher doses showed various complications which depended on the hydration status of the animal, its urinary flow and potassium level.