The high-amplification signal-averaged ECG (SAECG) allows the detection and analysis of late potentials (LP), which are related to a higher risk of severe ventricular arrhythmias in patients with myocardial disease and coronary artery disease. Because of excessive noise, this technique is usually performed at rest and occasionally immediately after interruption of exercise. Until now, however, it has not been used during exercise. The authors relate in this paper the method and results of the SAECG recorded at rest and during a standard stress test on 30 patients with a recent first myocardial infarction (day 10-14), in good condition, without ECG evidence of significant residual ischemia or bundle branch block. The SAECG was recorded during the steady state of a standardized stress test when a heart rate (HR) corresponding to 80% of the maximal HR was reached during a first test. Three patients had three criteria for LP, three others had two criteria, and two had only one. It has been impossible to analyze the SAECG during exercise because of excessive noise. One patient out of 30 in our study, 21 subjects had normal SAECG at rest and during exercise. None of the 6 patients with more than 1 abnormal criterion at rest became normal during the stress test despite a decrease of the duration criteria in all and an increase of the amplitude (root mean square) the terminal 40 ms (RMS40), as opposed to the 2 patients with just one positive criterion at rest (RMS 40 less than 25 muV) who became normal during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)