Seventy-two dog kidneys were stored under hypothermia as described by Collins and Sacks between 24 and 72 hr and then transplanted. The immediate function of the kidneys was measured by p-aminohippuric acid and inulin clearances. Twenty-four hr proved to be the maximum safe preservation time with both methods. The immediate function of the kidneys stored under hypothermia could not be improved by the addition of furosemide to the flushing solution. These results were compared with those gained by mechanical perfusion of the organ. Kidney function after 72 hr of hypothermic mechanical perfusion was significantly better than after 24 hr of hypothermic storage.