1. 1. In 50 dogs under full amytal anesthesia and before operative procedures, control electrocardiograms indicated marked normal variations of the T-waves. T 1 was positive in 13 instances, isoelectric in 10, diphasic in 2, and inverted in 25. T 1 negativity was usually accompanied by T 2 and T 3 positivity, but in several instances T 2 and sometimes T 2 and T 3 were also negative. Of the 25 dogs in which T 1 was negative, T 2 was positive in 6 instances, negative in 10, and diphasic in 9. T 3 was positive in 15 instances, diphasic in 4, and negative in 6. T 1 negativity was of variable magnitude, not infrequently sharply pointed and not unlike that described as typical of coronary occlusion. In only one instance was there any deviation of the S-T or RS-T segment of the electrocardiogram. 2. 2. Occlusion of the anterior descending branch of the left coronary artery is attended with characteristic electrocardiographic changes. In 26 of 33 dogs serially studied, deviations of the R-T segment were present, being pronounced in 9 instances and slight in 17 instances. These deviations are most conspicuous in the first two hours following ligation, that is, in the acute stage of myocardial infarction. These changes with associated negativity of the T-wave in Lead I are in accordance with the T 1 type of change as described by Parkinson and Bedford, Barnes and Whitten, and Barnes and Mann. 3. 3. Increased amplitude of T-waves, sharp negativity of the T-waves, particularly in Lead I, and successive directional and amplitudinal changes in the serial of electrocardiograms persisting as long as seventy-three days following ligation were noted. Low voltage of the initial ventricular deflection in Lead I was frequently encountered. 4. 4. Characteristic changes of rhythm were noted following coronary occlusion. Fifteen dogs developed ventricular fibrillation either immediately or within ten minutes following ligation. When ventricular fibrillation did not occur immediately, there was usually a period of normal rhythm for a variable number of hours, following which in eighteen recorded instances, there appeared premature contractions, usually both nodal and ventricular, rarely auricular. This was followed by nodal or ventricular tachycardia and later ventricular flutter, ventricular fibrillation, and death of the animal.