Repeat signal-averaged electrocardiograms were used in 40 patients to document changes in ventricular activation time and delayed potentials in the first 12 months after acute myocardial infarction. Beta-blocking and antiarrhythmic drug use was stopped for 1 week before each recording. Patients with reinfarction during follow-up were excluded. Signal-averaged electrocardiograms were first performed 1 to 4 weeks after infarction. They were repeated at 6 and 12 months in 31 patients in whom spontaneous ventricular tachycardia (VT) did not develop and were repeated after VT in the other 9 patients. Of the 9 patients in whom VT developed, all had delayed potentials at initial study and 8 (89%) still had delayed potentials at restudy after VT. In the VT patients, mean ventricular activation time was similar before and after VT (178 ms and 174 ms, respectively). In the 11 patients who had delayed potentials initially and in whom VT did not develop, the proportion with delayed potentials was 55% (6 of 11) at 6 months and did not change (55%) at 12 months. Mean ventricular activation time in these patients was 164 ms at initial study, decreasing to 147 ms at 6 months (p less than 0.05) and 148 ms at 12 months. In 20 patients with no delayed potentials initially, none had delayed potentials at 6 months and only 2 (10%) had delayed potentials detectable at 12 months. Mean ventricular activation time in these patients increased slightly, from 120 ms at initial study to 128 ms at 12 months (p less than 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)