Dogs breathed one of four gas mixtures (5% CO2-95% O2, 5% CO2-95% air, 10% CO2-90% O2, and 10% CO2-90% air) for as long as two hours. Regional cerebral blood flow as well as flow in nasal, otic, pituitary and skin tissue were measured by means of 15 ± 5 μ radioactively labeled microspheres. The normal values for cerebral blood flow and arterial blood gases were very similar to those of other investigators. Inhalation of CO2induced an increase in cerebral blood flow that was significantly higher than is usually reported. Increases varied from 100% (with 5% CO2-95% air) to 250% (with 10% CO2-90% O2). Blood flow in the temporal bone behaved much like that of brain in response to CO2. In most instances, the pituitary gland blood flow did not increase with inhalation of CO2.