Based upon scanning and transmission electron microscopy 3 regions are distinguished in the rat subfornical organ. The rostral region is dominated by nerve fibers interspersed with relatively few neurons and glial cells. Squamous to low cuboidal ependymal cells with flat ventricular surfaces bearing a few short microvilli line the center of this region; laterally, ciliated cuboidal ependymal cells predominate. The central region occupies the largest area of the organ and contains most of the neuronal perikarya and glial cells. Many perikarya and neuronal processes are located immediately underneath the ependymal surface. A dense capillary network with wide pericapillary spaces permeates the tissue. In the rostral two-thirds of this region the ependymal cells are either squamous or cuboidal, sometimes with slightly bulging ventricular surfaces bearing longer microvilli. Here supraependymal neurons are particularly numerous. The caudal one-third of the central region is characterized by squamous, cuboidal and columnar ependymal cells whose hemispherical ventricular surfaces are studded with long microvilli and occasional cilia and vesicular protrusions. The caudal region, like the rostral region, is dominated by nerve fibers between which neuronal perikarya and glial cells are present. At this level the choroid plexus is attached to the SFO through highly vascularized pial connective tissue. It is the major point of penetration of the SFO's capillary plexus. The possible significance of these observations and their importance in experimental interventions are discussed.