Plasma atrial natriuretic factor (ANF) levels were measured in 29 patients who had normal atrial hemodynamics on days 4 and 8 after an uncomplicated acute myocardial infarction (AMI). All patients had increased ANF levels on day 4. On day 8 of the ANF levels had declined significantly to normal values in 14 patients with enzymatically smaller AMI and higher ejection fractions (> or = 40%), while they remained elevated in 16 patients with larger AMI and lower ejection fractions (< 40%). The study data are compatible with increased secretion of ANF by the more extensively damaged left ventricles in AMI.