In order to elucidate the mechanism of shock or sudden death following intravenous calcium injection, which has frequently been used in the equine practice, the effect of this injection on the equine heart was investigated electrocardiographically. Three experiments were carried out with clinically healthy five horses. In experiment 1, 200ml of physiological saline solution was given intravenously as a control. In experiment 2, 2%, 5% or 10% aqueous solution of calcium chloride was given intravenously to study the effect of each concentration of the chloride. In experiment 3, to intensify the effect of calcium on the heart, a mixture of calcium solution and cardiac glycoside (20 ml of Rodealin^®, an extract from Rhodea japonica) was given intravenously. The horses were stabled without restraint during the experimental period. Electrocardiograms were recorded continuously by a radiotelemeter before injection up to 60 min after injection. The results obtained are as follows. In experiment 2, shortening of the Q-T intervals and the appearance of 2nd degree A-V block, ventricular premature beats or supraventricular tachycardia were observed. The severity of these findings correlated with the calcium concentration of the solution administered. In experiment 3, S-A block, A-V block, sinus tachycardia and supraventricular tachycardia were observed in the horses, which manifested shock symptoms. From these findings, it was presumed that acute cardiac insufficiency might be the cause of shock or sudden death of the horse following intravenous calcium injection.