Ultrastructural changes in rat hearts following cold cardioplegic ischemia of differing duration and differing modes of reperfusion. Morphologic consequences of prolonged global hypothermic (15 degrees C), cardioplegic ischemia and two reperfusion techniques were studied in Langendorff-perfused rat hearts. A 'gentle' reperfusion technique, with gradual rise in perfusate temperature and pressure to physiologic levels over 30 min, was used for 12 hearts following 2-hour or 3 1/2-hour (6 in each group) ischemia. Abrupt reperfusion, with perfusate at 37 degrees C and 70 mmHg, was performed on 13 hearts (6 ischemic for 2 hours and 7 for 3 1/2 hours). Six nonischemic, perfused hearts served as controls. Randomly selected specimens from the left ventricle after 45-60 min reperfusion were prepared for transmission electron microscopy. Volume fractions of myocardial structural components were calculated from stereologic point-counting on the electron micrographs. Two-way analysis of variance revealed that interstitial edema developed with increasing ischemic time and was not influenced by reperfusion technique. The degree of endothelial damage was independent of ischemic time, but was lessened by 'gentle' reperfusion. Both mitochondrial injury and myocyte edema were less when perfusate temperature and pressure were slowly raised after 3 1/2-hour ischemia. 