1. The tissue level of lysophosphatidylcholine (LPC) was determined in rat hearts perfused with a solution containing 5 microM LPC. The relationship between LPC accumulation and the severity of arrhythmias produced was examined. 2. The accumulation of LPC was dependent on the perfusion time and this accumulation was associated with the occurrence of severe arrhythmias. A positive correlation between the tissue LPC content and the arrhythmia score was found (P less than 0.01). 3. No consistent alteration in total phospholipid, phosphatidylcholine or cholesterol content was found. This suggests that LPC-induced arrhythmias are not associated with alterations of major lipid components in the heart. 4. When severe arrhythmias occurred in the presence of LPC in the rat perfused heart, less than 2% of total tissue phospholipid was in the form of LPC. 5. The positive correlation between LPC accumulation and the occurrence of arrhythmias suggests a cause and effect relationship of LPC with cardiac arrhythmias in the rat perfused heart. However, in the ischaemic heart, other biochemical factors can contribute, to different degrees, to ischaemia-induced cardiac arrhythmias.