Twenty-one patients were anesthetized by intravenous injection of thiamylal sodium and the relationships between the duration of anesthesia and the blood and interstitial fluid were studied. 1) A parallel correlation was seen between the anesthesia duration and the water content of plasma. 2) A reverse correlation was found between the sleeping time of anesthesia and quantity of the total plasma protein. 3) No interrelatiohships were found between the duration of anesthesia and hematerit, water content of blood, serum natrium, serum chloride, serum pottasium and plasma volume. 4) A reverse correlation was seen between the sleeping time of anesthesia and the interstitial fluid volume.