The relationship between the Q-oTc interval of the ventricular complex of calves and cows and the total calcium and calcium ion activity was quantitated. A proportional relationship of the Q-oTc existed between total calcium concentrations in the plasma of 5.5 to 8.2 mEq/L and calcium ion activities of 0.75 to 3.2 mEq/L. At higher calcium concentrations, the relationship was curvilinear and very variable. The Q-oTc interval was lengthened during induced hypocalcemic states and shortened during hypercalcemic states. Hypercalcemia seemed to decrease atrial activity and increase ventricular activity, as evidenced by the appearance of bradycardia, sinus arrest, and ectopic ventricular beats. Vagal activity seemed altered by the changes in plasma calcium concentrations and involved in the production of the arrhythmia, because atropine could abolish the arrhythmia created by the infusion of the calcium solutions.