The central nervous system of the cockroach has been incubated with solutions of an exogenous tracer substance, horseradish peroxidase, and the sites of its penetration and uptake have been studied by electron microscopy. When the nervous tissue is intact, or intact but stretched, the peroxidase is taken up throughout the neural lamella and also penetrates short distances into the extracellular space between adjacent perineurial cells. When the ganglia have been desheathed, reaction product for peroxidase is found in the neural lamella, perineurial cells, and within the cytoplasmic substance of the glial cells adjacent to the desheathed area. This uptake of peroxidase by the injured glial cells in desheathed preparations may reflect an alteration in the normal diffusion pathway from the external medium to the axonal surfaces.