Heart block is a potential complication in the surgical repair of several congenital cardiac defects. The proximity of the bundle of His to these defects makes their repair particularly hazardous. Earlier workers devised a method for the anatomic localization of portions of the conduction system at the time of cardiotomy. This method is inefficient and time-consuming and therefore has not received general acceptance. This paper describes an instrument which expands upon that method to facilitate and expedite the localization process. This should enhance the overall utility of the method. The instrument is self-contained, reduces the signal complexity arriving from the exploring probe, analyzes the waveforms and responds instantly with an audible tone when the probe overlies the bundle of His.