Storage in an intermediate host has permitted better and longer (5 days) preservation of a dog kidney than any other method. Perfusion with hypothermia, as well as hyperbaric oxygen and hypothermia, has in isolated instances permitted successful reimplantation after 3 days storage. Simple cooling is by far the simplest method and will by itself protect kidneys for at least 6 hours, and perhaps 24 hours with slight additions. Any perfusion technique, it seems to us, should use a perfusate without platelets or red cells. With such acellular perfusates, perfusions can routinely be continued for 3 to 40 days. Although no such perfused organ has been reimplanted successfully after more than 3 days, it seems likely that it suffered not so much from a lack of oxygen as from a lack of proper nutrients for energy and cell repair. Since this manuscript was submitted we have successfully reimplanted one canine kidney perfused for five days at 10 C with one part plasma: one part Eagle's solution and another kidney perfused for four days with just Medium 199; the membrane lung in both experiments was supplied with air; the kidneys are maintaining blood urea nitrogen levels at 51 and 57 mg% respectively.