In the course of an investigation into the nature of certain forms of damage to the brain from anoxia, cerebral air embolism was produced under controlled conditions in dogs, and was followed in 5 of these animals by intravenous administration of trypan blue. While this work will be reported in its entirety at a later date, the observations in the cases in which vital staining was involved are described here, in so far as they have a bearing on the subject of the hematoencephalic barrier. A general consideration of the physiologic factors involved precedes the description of the experimental phase of the work. Trypan blue was chosen as a dye which seemed to combine most of the essential qualifications for intravital use in a manner most suited to the purpose of this work. The fact that this dye will not, under ordinary circumstances, pass through the walls of vessels directly