To determine the optimum temperature at which the in situ kidney should be maintained while it is ischaemic, 47 mongrel dogs were studied. 35 of these underwent 90 minutes of left renal ischaemia with the kidney temperature maintained at 37 degree, 30 degree, 22 degree, 15 degree and 0 degree C respectively. The effect on renal function was determined by measurements of G.F.R. before and at regular 15-minute intervals after the inschaemic period. Computer statistical analysis exposed the optimum temperature to be 15 degree C. Renal artery blood flow, renal histology, 15-Cr labelled platelets and renal arteriography were used to determine the mechanism of ischaemic injury. Quantitation of renal cell injury confirmed that no additional protection to ischaemia could be gained by colling below 15 degree C. 15 degree C is recommended as the optimum temperature for use in clinical renal hypothermia.