A portable hypothermic perfusion system for storage of hearts has been developed. The system uses the airlift pump principle, whereby the flow of gas maintains circulation of the perfusate through the heart; no other energy source is required. Performance on ex vivo functional testing of 10 pig hearts stored for 20 to 24 hr using this system (group 3) was compared with that of freshly excised hearts (group 1) and hearts stored simply in the perfusate under hypothermic conditions, but not perfused (group 2). Group 2 hearts performed less well on functional testing than those of groups 1 and 3 which showed little statistical difference, suggesting good preservation by hypothermic perfusion. This has been confirmed by orthotopic transplantation of similarly preserved baboon hearts with survival until rejection at a mean of 27 days. The importance of the various constituents of the perfusate and the significance of weight gain during the storage and reperfusion periods are discussed.