There is evidence that the blood-brain barrier concept holds true not only for large molecules but for a great number of ions. Radioactive isotopes are not completely prevented from entering the internal environment of the central nervous system, but their passage from plasma to brain substance is reduced and delayed. However, the exchange of the same isotopes between plasma and cerebral lesions caused by trauma, irradiation, inflammation, vascular disorders, or neoplasms is more or less free. Consequently, a high concentration of radioactivity can be observed in lesions compared with surrounding normal brain tissue. Explanations of these phenomena can be arranged in two main groups. One theory lays emphasis on the increase of local capillary permeability, while the other identifies the selective barrier function with the function of cerebral tissue proper. When applied to pathological observations, this second theory would imply that metabolic rather than vascular changes within the lesion are