A system has been developed for the continuous hypothermic perfusion of isolated hearts using a clear fluid perfusate. Myocardial viability has been maintained after periods of storage of up to 48 hr. Pig hearts stored in this way showed almost normal hemodynamic performance on subsequent functional testing. Orthotopic allotransplantation or autotransplantation of baboon hearts stored for 48 hr was followed by good immediate and long-term function. Baboons receiving allotransplants survived until rejection. Three of four of those autotransplanted survived until electively sacrificed at 1, 3, and 12 months; all showed normal hemodynamic function on cardiac catheterization and normal myocardial histology. © 1986.