The presence of late potentials on the body surface recording was correlated with ventricular activation maps of reentrant circuits in the postinfarction canine model of reentrant excitation. Late potentials were found to correlate with delayed myocardial activation. However, during a reentrant rhythm complete diastolic activity on the body surface could not be detected if the mass of electrically active cells was too small and/or if very slow conduction in part of the reentrant circuit generated low amplitude extracellular potentials. Myocardial zones responsible for late potentials during a basic rhythm (e.g., sinus rhythm) may not necessarily be part of the critical zone of slow conduction during reentrant activation. Dynamic changes in late potentials are not amenable to temporal signal averaging techniques but could be detected by a high resolution beat-to-beat recording. A thorough understanding of the electrophysiological limitations of late potentials in the signal-averaged ECG could result in better utilization of the technique in clinical practice as well as in the development of new approaches for the detection of the arrhythmogenic substrate.