Three types of CO 2 titration curves were studied in the dog: (l) In vivo, arterial Pco, was increased by CO 2 breathing in the presence of high inspired oxygen. (2) In vivo, Pco 2 was increased by hypoventilation, while breathing air, producing hypoxia in association with respiratory acidosis. (3) In vitro, by equilibration of whole blood with CO 2 mixtures in oxygen. CO 2 titration curves obtained by Methods 1 and 3 confirm the findings of others that the in vivo curve differs from that found in vitro primarily because of the equilibration of CO 2 with interstitial fluid which has a lower buffering capacity than blood. The curve obtained by Method 2 differs from that of Method 1 since, beyond a Pco 2 of 60 mm Hg, bicarbonate concentration becomes progressively depressed, largely as a result of lactic acid formation secondary to hypoxia. This curve represents the effect of hypoxia on in vivo CO 2 titration and describes a relationship which must exist when severe respiratory acidosis develops during air breathing.