The reason for normal upright and negative T waves was examined by studies on the difference in durations of the action potentials (transmural gradient) in the epicardial (APDepi) and endocardial (APDend) surfaces. A unipolar epicardial electrogram (ECGepi) was recorded simultaneously with monophasic action potentials from both the epicardial and endocardial surfaces with suction electrodes in seven dogs. The duration of the action potential was altered by warming or cooling the epicardial surface of the heart. The T waves in the ECGepi were negative at room temperature in all experiments. When the epicardial surface was warmed, the T waves became less negative and finally became upright. Isoelectric T waves were obtained when APDend was 20 to 40 msec longer than APDepi. The amplitude of the T wave increased with increase in the transmural gradient (r value = 0.82 to 0.98). These results suggest that an APD of 40 to 60 msec is required for an upright T wave.