Experimental ischemic acute renal failure was made in 20 dogs by occulusion of the renal artery for one and half or two hours. Other 6 dogs were used for the control values of the distribution of the body water. 20 dogs, were raised under controlled intravenous infusion, and electrolytes, ureaN, osmotic pressure in both plasma and urine were measured for three days, and they were devided into two groups after three days according to the volume of the urine; 11 dogs, showing almost anuric (less than 2ml/kg/day)-anuric group, and 9 dogs getting urinary output more than 5ml/kg/day (average 20ml/kg/day)-urinary group. Total body water (TBW), extracellular water (ECW), intracellular water (ICW), interstitial water (ISW) and circulating plasma volume (PV) were measured with NAAP, (35)S-Na(2)SO(4) and (131)I-RISA. Results were as follows: 1. In the anuric group, there was no difference in TBW and distribution of body water from control values. 2. In the urinary group, TBW, ICW and U/P ratio of osmotic pressure and ureaN markedly decreased. 3. Pottasium, ureaN, osmotic pressure and total protein in the plasma increased in both groups, especially in the anuric group. 4. K/Na ratio of the urine in the urinary group, which is said to indicate the activity of aldosterone was not increased as proposed. 5. In the diuretic stage of the clinical acute renal failure, the water for massive urinary output surpassing fluid intake may be from the intracellular fluid compartment.