Pyrithioxine, a vitamin B6 derivative devoid of vitamin properties, causes a marked reduction in the permeability of the rat blood-brain barrier to phosphate while the blood-brain exchange of sodium is considerably increased under the influence of this substance. The effect of pyrithioxine was compared with that of pyridoxal and pyridoxine. It was shown in the mouse that under the influence of pyrithioxine the activity of C14-marked glucose injected intravenously becomes increasingly apparent in the brain. Possible clinical significance of these findings is discussed.