Knowledge of the normal physiology of the blood-brain barrier and of its alteration in diseases of the central nervous system constitutes one of the interesting advances in the field of neurology in recent years. Numerous studies on infectious, toxic, degenerative and post-traumatic diseases of the central nervous system indicate that the permeability of the blood-brain and the blood—cerebrospinal fluid barrier is more or less uniformly increased in these conditions.1 The blood— cerebrospinal fluid barrier has been shown to be impaired after such procedures as the pneumoencephalographic test, lumbar puncture, induction of spinal anesthesia, intrathecal therapeutic injection and ventriculographic examination.1 Technics involving such procedures have even been advocated to increase the efficacy of therapeutic agents which otherwise do not reach the central nervous system in effective concentrations.1 For the most part, these studies have involved measurements of the permeability of the blood—cerebrospinal fluid barrier, and the question has