Previously, rubidium-82 and positron emission tomography (PET) have been used to assess qualitatively the integrity of the blood-brain barrier (BBB). In this report, a method is described to measure the fractional extraction of rubidium across the BBB using the constant intravenous infusion of rubidium-82. In addition, the continuous inhalation of oxygen-15-labelled carbon dioxide is needed to measure regional CBF, and it is also necessary to measure regional cerebral blood volume. The assumptions underlying the model are identified, and the impact of deviations from these assumptions is described. From radiation dose and statistical considerations, it is shown that for normal cerebral tissue and the PET scanner used in this study, the rubidium extraction fraction can be quantitated only for regions as large as a cerebral hemisphere. However, quantitation is also possible for smaller regions where the focal extraction of rubidium is increased.