SHORTLY after the discovery was made that the central nervous system cannot be stained by intravenous injection of vital dyes and the concept of a blood-brain barrier was established, the question arose: Does or does not this barrier exist at birth? The phenomenon of kernicterus seemed to indicate that in the newborn infant the barrier is not yet fully developed, and experiments with vital dyes in young animals gave some support to this inference. The evidence these experiments offered was not certain, however. Moreover, some of these early investigations yielded contradictory results. It was thought that the application of radioactive phosphate would clarify the matter of the development of the blood-barrier and also reveal whether or not a similarity exists, in the intrauterine and the early postnatal stage, in the permeability of the barrier to pigments (bilirubin), vital dyes, and ions (P32). The analogy between the response of the