A new technique for 24-hr cardiac preservation is described utilizing very low flow perfusion (microper-fusion) with a cold flush solution. Rabbit hearts were arrested with UW solution and then perfused with the same solution through the aortic root at 0°C at a rate of 3–6 ml/gm heart weight/24hr. When tested on an ex vivo working heart model, the cardiac ouput (CO) was 28.72±7.69 ml/g/min compared with fresh UW flushed controls of 26.48±2.25 ml/g/min. Both oxygenated high-flow perfusion with a more conventional perfusate and 24-hr ice storage with UW led to inferior results. Omission of the colloid, hydroxyethyl starch (HES), from the UW solution or prolonged shelf storage were also significantly detrimental. When a previously untested colloid, polyethylene glycol 20,000, was substituted for HES for microperfusion, excellent cardiac function was obtained. In fact, the mean CO of this group, 31.91±5.70, was significantly above that of fresh HES-UW unstored controls. The suggestion that the UW solution might be improved by this substitution warrants further study.