A nonportable perfusion system, a cardioplegic agent, and a perfusate were developed for storing hearts by hypothermic perfusion for a period of 24 hours. The system and solutions were tested by biochemical and histological studies of the myocardium, by functional testing of the preserved pig hearts, and by orthotopic transplantation in baboons. When satisfactory storage for 24 hours had been achieved, the perfusion system was modified to facilitate portability. An airlift pump was used to provide both oxygenation and circulation of the perfusate. Consistent survival after allotransplantation and autotransplantation in baboons confirmed the reliability of this compact system. Four patients received donor hearts preserved by this system. Periods of ischemia ranged from 6 hours 55 minutes to 16 hours 50 minutes. The details of this experience are reported here.