The ultimate aim of our work is to understand the origin of body surface electrocardiographic potentials as they arise from cardiac depolarization. In two recent studies, we have found that cardiac fiber direction is a most important determinant of (1) the resistivity of cardiac tissue, (2) the strength of cardiac fibers as generators of current, (3) the velocity of depolarization, and (4) the potentials generated around a mass of depolarizing myocardium. These studies have been conducted on small volumes of myocardial tissue following epicardial stimulation. In the course of these studies, we have developed a theoretical model of cardiac tissue as a generator. We will now embark on a series of quantitative studies in which we measure depolarization pathways, use these with our tissue model to predict potentials during depolarization, and then compare these predicted potentials with those recorded during our experiments. We feel that these studies are most important to an understanding of the origin of the normal electrocardiogram and that it appears necessary to consider cardiac fiber direction in any understanding of the origin of electrocardiographic potentials.