Abstract
Horseradish peroxidase was implanted into rat forebrains and the distribution with respect to the periaxonal space was examined. The peroxidase filled the extracellular spaces. Usually, however, the flow stopped at the junction of the outer loop and the outermost lamella of the myelin sheath and at the outermost lateral loop at the node of Ranvier. Therefore, in most cases, the periaxonal space was devoid of peroxidase. Occasionally, however, peroxidase in small amounts evidently penetrated the junctions and could sometimes be clearly demonstrated within the periaxonal space.