Changes in the oxygenation properties and the relevant biochemical parameters of canine blood were followed during its storage in acid-citrate-dextrose (ACD) medium (4 degrees C) and after transfusion of the stored blood. Reductions of the blood P50 (pH 7.40, PCO2 40 Torr, 37 degrees C) and 2,3-diphosphoglycerate (DPG) during storage was much slower than in human blood. Half-decay time for DPG was 21 and 5 days for canine and human blood, respectively. The DPG decay could be extensively accelerated in the presence of bisulfite in ACD. The stored and DPG-depleted red cells required a considerably long period of time for the complete posttransfusional restoration of the DPG and P50. In five dogs whose blood were exchanged by 84% in average with the stored blood of mean DPG depletion of 88%, mean extents of the DPG and P50 restoration were 74 and 89% of the normal values after 24 h. After 5 days both the parameters were within the normal ranges. The red cell extra- along with intra-cellular pH fell immediately after the transfusion, and restored to the normal level after 24 h. From these results, correlations between P50 or red cell transmembrane pH gradient at the extracellular pH of 7.40 (delta pH) and red cell DPG/hemoglobin molar ratio (chi) were derived as P50 = 10.92 chi + 14.2 (r = 0.91) or delta pH = 0.060 chi + 0.135 (r = 0.70).