THE DETERMINATION of cerebrospinal fluid (CSF) protein concentration is one of the most frequently performed laboratory tests in the practice of neurology. While an elevated protein concentration is generally accepted as an indication of organic disease of the nervous system, little is known about the physiological mechanisms which normally preserve a CSF-plasma protein ratio of 1/200 nor of the pathological processes which lead to an elevated protein concentration. A dynamic equilibrium between plasma and CSF albumin was first shown to exist in the dog by Fishman1 and was subsequently shown to exist in man by Frick and Scheid-Seydel.2 These workers demonstrated that plasma albumin is the precursor of CSF albumin, and that radioiodinated proteins were catabolized at approximately the same rate in both plasma and CSF compartments. A steady state CSF-plasma protein ratio of 0.005 implies that the fractional rate of removal of protein from the CSF