The use of a digital display automatic chronometer during epicardial mapping is reported. The epicardial potential recorded by a tripolar electrode is transformed electronically by amplification, filtering, rectifying negative deflections and then summation. The interval between the reference signal (electrical impulse or other cardiac potential) is calculated automatically and displayed at an adjustable rate. The presence of a delay increases the range of application of the apparatus. Epicardial mapping is thus simplified and shortened.