THE OCCURRENCE of cerebral vasodilatation and vasoconstriction in response to elevation and depression of blood carbon dioxide tension has been well established.1-4 The time required for achievement of the full cerebral vasodilator response to carbon dioxide inhalation remains unknown because of methodological limitations. In 1933, Gibbs5 described a thermoelectric device designed to follow continuously changes in blood flow in the jugular bulb, and by 1935, Gibbs and associates6 reported the instantaneous effects of alterations in blood gases on flow as detected by this probe. These results have been considered qualitative by these authors7 and others8 because of lack of in vivo calibration and other technical problems. The subsequent development of the nitrous oxide method by Kety and Schmidt9 allowed the first quantitative measurements of human cerebral blood flow (CBF). Since this method requires 10-14 minutes for a single flow determination, it cannot