The change in oxygen tension (pO2) of shed blood was determined polarographically for 2 hr after shedding at temperatures of 37.5, 22—24, and 1 C. The results show that blood pO2 decreases progressively after shedding, that the decrease is especially marked in blood of high initial pO2 values, and that cooling of the sample slows but does not stop the metabolic processes leading to oxygen utilization. The results reaffirm the need for rapid analysis of blood for pO2. polarographic pO2 analysis; temperature effect; blood metabolism Submitted on March 6, 1964