A large quantity of blood was shed from dogs and immediately replaced by the injection of an equal volume of Acacia-Ringer solution (Ringer solution mixed with 7% of gum acacia) into the vein in order to prevent any fall of blood pressure, (due to loss of blood, ) and to maintain the normal state of circulation. The content and tension of oxygen in the blood under the abovementioned conditions and the recovery of these from their now lowered level to the normal, were studied. Concomitant changes in the blood, viz., in the number of erythrocytes, and in the volume-ratio of blood corpuscles to the plasma and the haemoglobincontent of blood, were observed during the successive days. 1) By the letting of blood, the oxygen content in the arterial blood was lowered until it was 8.95 vol% or below; in these cases the animals succumbed always; but in cases in which the oxygen content remained over 10.55 vol%, the animals survived. 2) The critical amount of oxygen in the blood, the severe loss of blood and the lowered oxygentension of the respiratory air, all coincide in producing death. The determining factor in producing death, in these cases, was the deficiency of oxygen in the blood. 3) The decrease of the oxygen content in the blood below the critical value is shown by the decrease of the respiration frequency, especially in the most case by the respiration of Cheyne-Stokes type. 4) It seems as if failure of the respiratory centre to respond foretells the death of animals in the case of anoxaemia, for if the frequency of the respiration in anoxaemia exceeds the normal rate (i.e. the rate of respiration before the bleeding a. s. f., ) the prognosis is generally favourable; but if the centre can not respond to the lack of oxygen with increased respiration frequency, the animal, without exception, expires. 5) The amount of the lost blood until about 44% of the total blood volume (which was calculated as 9.72% of the body weight, according to Meek and Gasser) could be safely replaced by Acacia-Ringer solution; but a loss of blood exceeding 45% could not be safely replaced in this way. 6) Socalled “head of oxygen” i.e. the difference between the oxygencontents of arterial and venous blood decreased to 2.0% (normal value is 5.66 to 3.66%, average 4.26%, ) when about 44% of blood was replaced by Acacia-Ringer. If this value fell lower than 1.62%, by the replacement of a greater amount of blood by Acacia-Ringer, the animal died. 7) Complete recovery of the blood to its normal volume and number of bloo-dcorpuscles, required 33 to 40 days, if the shed blood was replaced by an equal amount of hypertonic Acacia-Ringer. If isotonic Acacia-Ringer was used, the recovery was effected in 24 to 27 days. Recovery of the amount of haemologlobin delayed about 3 days from this term. 8) The recovery of the blood to normal conditions of oxygencontent, numbers of erythrocytes and the amount of haemoglobin proceeded steadily, when replacement of the shed blood was made by isotonic Acacia-Ringer. If hypertonic Acaeia-Ringer was used for replacement, hydraemia followed withinin a few days during the early stages of recovery.