&NA;The area postrema is a circumventricular organ that plays an important role in neurohumoral regulation of the circulation. We have developed a method to examine permeability and vascular responses of the microcirculation of the area postrema in vivo. A craniotomy was performed over the dorsal brain stem in anesthetized rats, and blood vessels to the area postrema were visualized with fluorescent microscopy. Extravasation of sodium fluorescein (MW, 386), but not 150 kDa (MW) fluorescein isothiocyanate‐dextran, occurred in the area postrema under control conditions. There was no extravasation of fluorescein or dextran in the brain stem under control conditions. Acute hypertension produced marked disruption of the barrier to 150 kDa dextran in the area postrema, compared with minimal disruption in the brain stem. We tested the hypothesis that the area postrema has greater permeability to small molecules than the brain stem and that this permeability might be accompanied by distinctive vascular responses. Topical suffusion of adenosine and ADP produced similar dose‐related dilation of arterioles to area postrema and dorsal brain stem. Topical and intravenous vasopressin produced similar dose‐related constriction of vessels to area postrema and brain stem. Electron microscopy in rats demonstrated that a barrier to horseradish peroxidase, which is absent in capillaries in the area postrema, is present in arterioles that supply the area postrema. Thus, 1) the microcirculation of the area postrema is permeable to relatively small molecules under normal conditions and is more susceptible than the brain stem to disruption of the barrier by large molecules during acute hypertension and 2) regulation of vascular tone in response to several stimuli is similar in area postrema and brain stem, despite marked differences in capillary permeability. (Circulation Research1989;65:417‐425)