We evaluated high-energy phosphate (HEP) levels and the ability to perform work in rat hearts preserved by standard techniques (0.9% NaCl arrest and storage at 4 degrees C) and by continuous coronary perfusion at 22 degrees C, pH 7.25, and 55 mm Hg for 4 or 8 hr with Krebs-Henseleit buffer (KHB), modified Morgan's solution (MCS), or a medium developed in our lab (physiologically complete medium or PCM). Cardiac work was evaluated in the rewarmed hearts with use of a working heart preparation at left atrial pressures of 10, 15, and 20 cm H2O, and by measurement of aortic flow, coronary flow, heart rate, and peak systolic pressure. HEP levels in the hearts continuously perfused were significantly higher (p less than .05) than those in the hearts stored at 4 degrees C. The functional recovery of hearts preserved by storage in cold saline for 4 or 8 hr was significantly less (p less than .01) than the recovered function of hearts preserved for comparable periods by perfusion at 22 degrees C with either MCS or PCM. The results indicate that continuous perfusion at 22 degrees C with a more physiologic medium is superior to hypothermic arrest and storage at 4 degrees C for the preservation of donor heart function and HEP levels.