50 dog kidneys were mechanically perfused under hypothermia for between 12 and 72 h and then transplanted. Before perfusion the kidneys were subjected to 15,30,45 and 60 min of warm ischaemia: 15 min of warm ischaemia were well tolerated (successful 72-h preservation), but after 30 min preservation was successful for only 24 h. Enzyme release and lactate formation depended on the duration of warm ischaemia. Furthermore, these perfusate concentrations indicated the viability of the organ (highly significant differences between well functioning kidneys and those with little or no function.