Chronically implanted polarographic electrodes were used in cats to measure regional oxygen availability (O2a), from which blood flow was inferred. Occlusion of the middle cerebral artery at its origin regularly caused an abrupt fall in O2a which was greatest in the sylvian region near the occlusion and least at the occipital pole. Following occlusion, circulatory dynamic responses to CO2 inhalation were studied. These were a paradoxical fall in O2a during hypercarbia, most commonly occurring in the sylvian region, and passive responsiveness to blood pressure in intermediate regions. In some cases, at the end of CO2 inhalation, an increase in O2a above the preinhalation level occurred which was independent of blood pressure. In chronically surviving animals, the paradoxical response to CO2 usually disappeared and was replaced by lesser abnormalities or normal responses. The fundamental principle determining the response to hypercarbia in cerebral vascular occlusions appears to be the competence of the collateral circulation supplying the ischemic region. © 1970 American Academy of Neurology.