Mean and phasic coronary flows were recorded simultaneously by an electromagnetic meter, and peripheral coronary pressure was recorded distally from the temporarily occluded descending branch of the left coronary artery. From the results of these studies we conclude that: 1. The sympathetic cardiac nerves, as does levarterenol, increase myocardial contraction, (indicated by a shorter, more abrupt systole and more rapid isometric relaxation), and cause a slight degree of coronary arteriolar dilation. 2. Parasympathetic fibers exert no significant effect, although acetylcholine diminishes coronary arteriolar tone and may slightly diminish ventricular contraction. 3. That measurements of mean coronary flow and resistance are relatively satisfactory for indicating changes in coronary arteriolar tone, but are of relatively little value in evaluating myocardial contraction.