Young dogs of both sexes were used in this study. Transmission and scanning electron microscopy were utilized for the examination of the spinal cord and choroid plexus with emphasis on the study of free cells. These procedures were modified so that, in certain cases, the same cells observed in scanning electron microscopy could be analyzed internally by transmission electron microscopy. One half of the animals were injected under anesthesia with horseradish peroxidase for observation of phagocytosis. This study confirms that the free cells observed in the subarachnoid space with the scanning and transmission electron microscopes are identical. The internal morphology of these cells corresponds to that of macrophages. This is further substantiated by the ability of these cells to localize horseradish peroxidase in discrete vacuoles within their cytoplasms. Both pial macrophages and epiplexus cells localize peroxidase in an identical manner in the same animal after one injection. In addition macrophages on the surface of the pia mater respond to extravasated red blood cells in a characteristic manner including phagocytosis. The plentiful population of macrophages on the surface of the pia mater supports the concept that these cells are of major importance in maintaining asepsis in the subarachnoid space.