This study has been suggested by the results obtained in a previous investigation of the behavior of calcium in blood during sleep induced by diallyl-barbituric acid.1The question has arisen whether the diminution of the calcium content in blood observed in most experiments performed during sleep was accompanied by any corresponding modifications of the calcium content of the cerebrospinal fluid. Moreover, it appeared worth while to compare the measurements of the fluid calcium with those of the blood calcium; that is, to study the fluid calcium-blood calcium ratios in the light of the following concepts: (1) the idea of Cloetta and Thomann,2according to which the diminution of calcium in blood during sleep is associated with a shift of calcium from the blood to the brain; (2) the dominant view on the function of the hemato-encephalic barrier and the rôle of the cerebrospinal fluid; that is, that the