Examination of the squirrel monkey (Saimiri sciureus) area postrema (AP) revealed this circumventricular organ to be primarily composed of two types of glial cells and a single type of neuronal element. No pattern of neuronal arrangement could be discerned, however, this cell type was frequently observed in close relation to the perivascular spaces. The neuronal elements, although slightly larger than the glial cells, were characteristically less electron dense. The neurons routinely displayed an infolded nuclear membrane, a single nucleolus and the normal complement of subcellular organelles. Synaptic terminals were numerous, and both axo-somatic and axo-dendritic varieties were observed with the latter being more numerous. Both clear-cored and dense-cored vesicles could be observed in the same ending. Unmyelinated neuronal processes were the predominant type within the interior of the AP, although myelinated processes were also regularly present. Non-neuronal elements with the AP resembled CNS astrocytes and were as numerous as the neuronal elements. This cell type appeared to envelope completely the vasculature and separated the parenchyma from the perivascular spaces. The ventricular surface of the AP was covered by modified ependyma which lacked kinocilia but frequently demonstrated microvillar projections. Opposed ependymal cell membranes showed interdigitations, and zonula adherens-type cell junctions connected the ependymal cells near the ventricular lumen. Two types of bulbous projections were observed in the ventricular lumen close to the ependymal surface. The most characteristic feature of the AP, however, was its vascularity. Perivascular spaces surrounding fenestrated capillaries contained fibroblasts and collagen. The vascular endothelium routinely demonstrated pinocytotic activity, and the basal lamina was prominent.