Although there is general agreement that renal blood flow is depressed by body cooling, there is some disagreement concerning the extent of this alteration. These discrepancies may be related to the wide variation in the degree of hypothermia induced1,6,7,10,11,15and, more particularly, to the methods for measuring renal blood flow.13Some evidence has been presented in the literature indicating that the conventional clearance techniques for measuring renal plasma and blood flow are not necessarily reliable in the hypothermic animal. Harsing and his associates6have pointed out that the clearance of para-aminohippurate may not be complete under hypothermic circumstances, possibly as a result of repression of secretory function of the tubules of the kidney. Balint and his colleagues2have recently pointed out that this may also be true when the extraction rate is simultaneously determined. Because of these discrepancies and the fact that we have been unable