[long dash]The concentration of Na24 and specific activity of plasma, cerebrospinal fluid, and various parts of the brain were determined in cats by radioautography and direct tissue analysis 8 minutes to 5 hours after intravenous and intracisternal injection, respectively. After intravenous injection, 50% of the plasma Na24 concentration was attained in the cerebrospinal fluid in 1 hour. Complete equilibrium was not reached by 5 hours. The Na24 concentra tion of brain tissue revealed a slow but steady increase with time. There is a considerable delay in Na equilibration between plasma and cerebral tissue compared with other tissues, due to the blood-brain barrier effect. Superimposed on the direct capillary exchange of Na is evidence of additional diffusion from the cerebrospinal fluid. The exchange of plasma and brain Na is already restricted at the midtime of pregnancy. The pathways and rate of absorption of Na24 by the brain tissue after intracisternal injection are presented. The absorption from the cere-brospinal fluid is faster than from the plasma when equal mixing of the tracer in the cerebrospinal fluid is provided for and when the data are correlated to identical concentration in the respective fluids and to absorptive surfaces of equal size. Considerably higher amounts of the isotope were found in the injured area after intravenous injection than in the normal brain. This local increase in the permeability of the blood-brain barrier was greatest immediately after injection. It decreased rapidly with the lapse of time, but was still noticeable 5 hours later. The Na24 content of the lesion is directly related to the prevailing plasma concentration. Na was not taken up selectively by the lesion when applied directly over the cerebral cortex. When the rate of cerebral uptake and the regional distribution of Na24; a primarily extracellular ion, is compared with that of an intracellular ion, as represented by P32; it reveals that the difference involves only the temporal but not the spatial aspects of their absorption.