1. Sodium fluorescein and Evans Blue, commonly used tracers in the study of blood-brain barrier disturbances, revealed considerable differences in their respective protein binding capacity in the plasma, passage through the barrier and in the rate of their elimination from the brain parenchyma. 2. In the plasma a considerable portion of the sodium fluorescein remains free and behaves like a micromolecular barrier tracer. On the other hand, almost complete binding of the Evans Blue to albumin confers to it properties of a protein tracer. 3. Following the extravasation of the tracers, the sodium fluorescein is relatively soon eliminated, whereas Evans Blue remains in the cellular elements of the brain parenchyma for a considerable time, although the protein moiety of the tracer is removed much sooner from the cytoplasm of glial cells, presumably by the lysosomal digestion.