Within the framework of a new concept for ex vivo conditioning organs prior to transplantation, prostaglandins F2α (PGF2α) and E2 (PGE2) were investigated for their protective effects against in vitro immunologically induced morphological changes in the myocardium. Experiments were carried out with isolated rat hearts preserved at + 4 °C for 6 days with a ‘cytoplasmic’ perfusion fluid in the presence of an anti-rat serum and/or either PGF2α and PGE2. Electron microscopical investigations have shown that prostaglandins are able to prevent or to minimize remarkably myocardial lesions induced by the anti-ra