Oxygenated fluorocarbon cardioplegic solution was tested in 20 isolated rabbit hearts. In 10 of the hearts, 150 ml of solution (Fluosol DA) with addition of 20 mmol/l potassium, 4 degrees C, was infused during 3 min into the aortic root (infusion pressure 60-80 mmHg). The other 10 hearts were perfused with Fluosol DA containing 20 mmol/l potassium and 15 mmol/l magnesium. The hearts were left immersed in the perfusion solution for 2 hours. Myocardial biopsies were performed while the hearts were still beating, immediately after completion of the cardioplegic infusion and 60 and 120 min later. Transmission electron microscopy showed progressive, time-related deterioration of the myocardial cells, characteristic of myocardial ischaemia, in both groups. The changes, however, were clearly less pronounced in the K/Mg-Fluosol DA group. Myocardial capillaries remained patent in both groups throughout the observation period, but the capillary endothelium was better preserved in the K/Mg-Fluosol DA group. The study thus indicated that K/Mg-Fluosol DA was superior to K-Fluosol DA as a cardioplegic agent in terms of ultrastructural preservation of the myocardial cells and the endothelial lining of the myocardial capillaries.