Experiments were designed to determine the viability of endothelial cells and their responses to products released by aggregating platelets following single flush perfusion of the coronary arteries with cardioplegic solutions used for cardiac transplantation. Porcine coronary arteries were perfused with crystalloid (Plegisol®) or blood cardioplegic solutions: nonperfused hearts placed in 0.9% saline were used as controls. Immediately following perfusion and after 5-hour storage of the hearts in the same cardioplegic solution, rings were cut from the right coronary arteries and suspended in organ chambers for the measurement of isometric force. In some rings the endothelium was removed deliberately. The left circumflex coronary arteries were flushed with collage-nase and the harvested endothelial cells were plated for cell culture. Left anterior descending coronary arteries were perfusion fixed with glutaraldehyde for scanning electron microscopy. In the organ chamber experiments, aggregating platelets and adenosine diphosphate caused relaxations in rings with endothelium. These relaxations were reduced immediately following crystalloid cardioplegia and were restored following 5-hour storage. Serotonin caused contractions in all rings. Rings without endothelium were more sensitive than rings with endothelium to the amine; this difference was augmented following 5-hour storage of the heart. Significantly fewer foci of endothelial cells grew in culture following 5-hour storage of the hearts in crystalloid cardioplegic solution compared to control (p < 0.05). There were no anatomical differences identified among groups by scanning electron microscopy. These results suggest that crystalloid cardioplegia alters the responses of coronary arteries to substances released by aggregating platelets and reduces the ability of endothelial ceils to replicate. Such changes may contribute to altered vascular resistance following reperfusion of transplanted hearts and potentially to later structural changes in the coronary arteries. © 1993 S. Karger AG, Basel.