Sympathomimetic drugs infused into the left anterior descending coronary artery of unanaesthetized dogs produce two distinct electrocardiographic responses. Compounds with two or three hydroxyl groups cause depressed ST segments and tall peaked T waves of precordial leads. Agents with one or no hydroxyl group, or with an oxymethyl group, cause coved ST segments and inverted T waves. The tall peaked T wave elicited by noradrenaline is associated with an increased rate of fall of phase 3 of the monophasic action potential of openchest animals, whereas the inverted T wave is associated with a decreased rate of fall of phase 3.