A system has been developed for the ex vivo functional testing of isolated hearts. Three groups, each of 10 pig hearts, have been studied: group 1 - freshly excised hearts; group 2 - hearts hypothermically perfused for 20-24 hours with Krebs-Henseleit solution; group 3 - hearts similarly perfused with a clear fluid hyperosmolar solution. Group 2 hearts performed poorly on functional testing and would clearly have been unsuitable for transplantation. Haemodynamic observations on group 3 hearts showed little statistical difference from those of group 1, suggesting good preservation. The value of the ex vivo testing system as a reliable means of assessing myocardial function is discussed, and has been confirmed by subsequent successful orthotopic transplantation of baboon hearts stored under identical conditions.