The dynamics of signal-averaged ECG and late potentials was studied in 19 patients with coronary heart disease during spontaneous anginal episodes. Baseline late potentials were observed in 9 (45%) patients, they remaining in all during and after anginal episodes. The occurrence of new late potentials was recorded in none of them. There were no significant differences in some parameters of signal-averaged ECG before, during, and after an anginal episode, though the duration of low-amplitude signals tended to increase at the end of the QRS complex and the mean square amplitude of late 40 msec of QRS complex decreased during transient myocardial ischemia. The differences did not achieve their statistical significance. It was concluded that transient myocardial ischemia during spontaneous anginal episodes failed to lead to the appearance of a substrate for the occurrence of late potentials.