Fifty-six patients with acute transmural anterior wall myocardial infarction (MI) were investigated with a 24-electrode grid and 34 patients with an acute transmural inferior wall MI were investigated with standard ECG leads II, III, and aVF in order to study the length of time after the onset of pain during which the development of Q waves and reduction of R waves progress. These ECG changes continued for 18-26 h after onset of pain but the majority appeared during the first 12 h.