Investigations on the relative degrees of the passage of different substances from the blood into the cerebrospinal fluid have led to the assumption that a barrier exists between these two fluids. This was based on the experimental observations that substances injected into the blood stream do not necessarily reach the spinal fluid in the same proportions as they are found in the blood. This discovery of what appeared to be a mechanism regulating the passage of different substances from the blood into the cerebrospinal fluid led to further research by physiologists as well as by those whose interests were more along clinical and therapeutic lines. The physiologist was interested primarily in the nature and localization of this mechanism. The clinician, however, searched for possible changes in this mechanism in different pathologic conditions, as well as for methods whereby it could be influenced for therapeutic purposes. In the present communication we