The blood-brain barrier (BBB) prevents entry of circulating substances into the brain. The circumventricular organs (CVOs) lack a BBB and have a direct communication with the circulation blood. One of the CVOs, the area postrema (AP), which has a close relationship with the nucleus of the tractus solitarius (NTS) and dorsal motor nucleus of the vagus nerve (DMX), plays a role in controlling the entry of blood-borne substances to neurons of the brainstem. To clarify the cellular localization of protein components of the BBB in the brainstem AP-NTS region, we used antisera to--(1) Tight junctions: claudin-5 and zona occludens-1 (ZO-1). (2) Endothelial cells: (a) all endothelial cells--rat endothelial cell antigen-1 (RECA-1) and (b) endothelial cells at BBB--endothelial barrier antigen (EBA), glucose transporter 1 (GLUT1) and transferrin receptor (TfR). (3) Basal lamina--laminin. (4) Vascular smooth muscle cells--smooth muscle actin (SMA). (5) Pericytes--chondroitin sulfate proteoglycan (NG2). (6) Glial cells: (a) astrocytes--glial fibrillary acidic protein (GFAP), (b) tanycytes--dopamine- and cAMP-regulated phosphoprotein of 32 kDA (DARPP-32), and (c) microglia--CD11b. Neuronal cell bodies in the NTS were visualized by antisera to neuropeptide Y (NPY) and alpha-melanocyte-stimulating hormone (alpha-MSH), two peptides regulating energy balance. This study provides a detailed analysis of the cellular localization of BBB proteins in the AP and NTS and shows the existence of vessels in the dorsomedial aspect of the NTS that lack immunoreactivity for the BBB markers EBA and TfR. Such vessels may represent a route of entry for circulating substances to neurons in the NTS that inter alia regulate energy balance.