Fifty-seven patients, all with chronic respiratory insufficiency and hypoxemia, have been classified in five groups according to their acid-base status, determined by the intra-erythrocytic pH (pHi) : A. normal acid-base balance ; B. chronic alkalosis ; C. acute alkalosis ; D. acute acidosis ; E. chronic acidosis. We have measured in the arterial blood : Po2, PCO2, plasmatic pH (pHp1), pHi, P50(7,40) and 2,3-DPG. From these values. the base-excess (B.E.) and the P50 to real pH [P50 I.V.] have been computed. There is a good correlation between 2,3-DPG and P50 (r = 0.707), none between P50 and Pao2. For a normal acid-base balance, P50 and 2.3-DPG also are normal. Increased in chronic alkalosis, the amount of DPG is decreased in chronic acidosis. The DPG-pHi correlation is very good (r = 0.691 ; pless 0.001) and the changes of the acid-base balance seem to be the main factors for controlling the DPG synthesis : it increases it in chronic alkalosis and reduces it in chronic acidosis. Thus the P50(I.V.) returns to the normal range on account of the Bohr effect adjustement. On the contrary, in acute acidosis and alkalosis, the amount of DPG is normal and the P50(I.V.) is increased or reduced. Therefore the duration of the acid-base lack of balance also interfers.