To elucidate the mechanism of reduction in the renal concentrating operation in hypothermia, the kidney were removed from 13 euthermic and 10 hypothermic (average rectal temp. 25C) dogs and four slices of the tissue from papilla to cortex were cut for each kidney. The Na, K, Ci and urea concentrations of the slice were then determined in slice extracts. Although urine osmolarity was significantly reduced in hypothermia, the overall relationship between papillary Na and urea concentrations and urine osmolarity was comparable in both euthermia and hypothermia, indicating that the renal concentrating operation is impaired in hypothermia primarily because of corresponding reduction in the medullary osmotic gradient. However, papillary Na and urea concentrations at a given urine osmolarity was somewhat higher in some hypothermic kidneys than in euthermic ones, suggesting that the impairment of renal concentrating ability in hypothermia is partly attributable to a failure of the collecting duct to osmetically equilibrate with the surrounding media. On the basis of the latter finding, it is postulated that
the action of ADH is less effective in hypothermia, and the possible reasons for this postulate are discussed.