The growing interest in disturbances of calcium metabolism within recent years has stimulated investigation of the state of calcium in blood serum and its distribution in various body fluids. The cerebrospinal fluid has received particular attention since, if it is assumed that this fluid is a protein-free filtrate of blood plasma, its calcium content may be taken to represent the diffusible calcium of the blood. The question arises as to whether the cerebrospinal fluid may be correctly regarded as a protein-free filtrate of blood plasma. The literature on this point has been extensively reviewed by Becht,1Weed2and Levinson.3Several arguments have been advanced in support of the belief that the fluid is formed by a secretory process. Petit and Girard4demonstrated an increased flow following the administration of pilocarpine, muscarine and ether. Cappilletti5found a diminished flow following the use of atropine and hyoscyamine. Similar results have been obtained