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To obtain information concerning the time course and instantaneous distribution of the excitatory process of the normal human heart, studies were made on isolated human hearts from seven individuals who died from various cerebral conditions, but who had no history of cardiac disease. Measurements were made from as many as 870 intramural terminals. In the isolated human hearts three endocardial areas were synchronously excited 0 to 5 msec after the start of the left ventricular activity potential. These areas increased rapidly in size during the next 5 to 10 msec and became confluent in 15 to 20 msec. The left ventricular areas first excited were (1) high on the anterior paraseptal wall just below the attachment of the mitral valve; (2) central on the left surface of the interventricular septum and (3) posterior paraseptal about one third of the distance from apex to base. The last part of the left ventricle to be activated usually was the posterobasal area. Endocardial activation of the right ventricle was found to start near the insertion of the anterior papillary muscle 5 to 10 msec after onset of the left ventricular cavity potential. Septal activation started in the middle third of the left side of the interventricular septum, somewhat anteriorly, and at the lower third at the junction of the septum and posterior wall. The epicardial excitation pattern reflected the movements of the intramural excitation wave. Conduction velocity was determined in one heart by an appropriate stimulation technic. Atrial excitation, explored in two hearts, spread more or less according to concentric isochronic lines. Control studies, carried out on five canine hearts, disclosed that the pattern of ventricular excitation did not change after isolation and perfusion. However, total excitation was completed earlier in the isolated heart, and conduction velocity increased. Careful mapping illustrations are presented.