The area postrema of the monkey, Macaca fascicularis, were a pair of oval organs at the caudal end of the floor of fourth ventricle. Their ependymal lining was covered by well-developed microvilli with occasional overlying supraependymal cells. Two types of lining cells were present: pyramidad- and flattened cells. The pyramidal cell showed a long extending basal process resting on the underlying blood vessels. In transmission electron microscopy, the organ showed numerous fenestrated sinusoids characterized by a distinct perivascular space containing mast cells, macrophages and collagen fibrils. The parenchyma of the organ was composed of neurons and glial elements. Only one type of neuron ranging from 9.5 to 15 microns could be distinguished. The neurons contained an indented nucleus surrounded by organelle rich cytoplasm. The soma of the neuron was enclosed by glial element resembling astrocyte. The glial processes terminated on the blood vessel where they were "tunnelled" by a variable number of nerve fibres some of which gained a direct access to the external basal lamina of the perivascular space. Synapses in the neuropil predominantly of the axodendritic variety were observed. Axon terminals containing round agranular vesicles were seen to make synaptic contacts with the neuronal soma. No structural changes were observed in the area postrema following bilateral cervical vagotomy. However, degenerating axon terminals were observed in the subpostremal zone 7, 14 and 21 days after vagotomy suggesting a direct afferent projection into this region.