We investigated the relationship between platelet aggregability and occurrence of complex arrhythmias in 72 males in the third or fourth week after the onset of myocardial infarction. The patients who displayed complex arrhythmias (n = 31), as opposed to those with stable rhythm (n = 41) showed higher platelet aggregability (48 +/- 27% vs 37 +/- 22%, mean +/- standard deviation, respectively, P less than 0.01). No differences were found with respect to plasma free fatty acids and excretion of catecholamines. It is suggested that, after myocardial infarction, increased platelet aggregability may contribute to the development of complex arrhythmias.