SUMMARYElevation of serum enzymes often accompanies tissue damage in vivo. The aim of this study was to determine whether the output of the enzyme lactate dehydrogenase (LDH) during a 15-min perfusion of an isolated kidney was a measure of acute ischaemic renal damage. The results indicated that, in general, LDH output increased with increasing time of total renal ischaemia. Also, the duration of warm ischaemia and LDH output of the excised kidney were compared as indices of renal damage. This damage was assessed by the postoperative function of the remaining kidney which had undergone the same ischaemia as the one excised. The LDH output of the excised kidney showed the closer correlation with subsequent renal function