Effects of operation on seruml evel and urinary excretion of sodium, chloride and potassium, and water and electrolytes coutent in tissue were studied on dogs in both hypothermic and normothermic conditions. Following results were obtained. 1) Hyponatremia and hypochloremia, decrease of urinary excretion of both electrolytes, increase of potassium-sodium ratio and increase of urinary excretion of potassium in accordance with decrease of urine volume were found during postoperative period in dogs who underwent operation under normothermia. However, increase of urine volume occurred on about 5th or 6th postoperative day resulting in increase of urinary excretion of sodium and chloride, and decrease of potassium excretion. Increase of water, sodium and chloride content of the tissue which had operative intervention, were found on the 4th postoperative day, though decrease of potassium was noted. 2) Decrease of serum sodium and chloride, and increase of potassium were noticed in hypothermic group, though these changes were not so significant as in nomothermic group. Urinary excretion of sodium and chloride increased in accordance with increase of urine volume and chloride-sodium ratio were diminished. However, changes after 5th to 6th postoperative day were same in both groups. There were found no significant changes in water and electrolytes content of tissue, which had operative intervention, on the 4th postoperative day in hypothermic group. Increase of water, sodium, and chloride in tissue were slight and no decrease of potassium was found. It was anticipated based on these findings that excretion of ADH and function of adrenal gland were suppressed resulting in increase of urinary excretion of sodium and chloride with less retention of water, sodium and chloride in tissue during postoperative period in operation performed under hypothermia. In addition to, decrease of potassium content in tissue was not found, suggesting of less consumption of body protein and suppression of catabolism during postoperaetive period.