The ultrastructural morphology and specialized neuronal, vascular, and ventricular associations of tyrosine hydroxylase-labeled neurons are examined within the area postrema of rat brain. Specific antiserum to the purified enzyme is localized throughout the rostrocaudal and dorsoventral extent of the area postrema by means of the peroxidase-antiperoxidase technique. In all regions, peroxidase immunoreactivity for tyrosine hydroxylase is distributed throughout the cytoplasm of selectively labeled neuronal perikarya and processes. The perikarya contain a large nucleus, infolded nuclear membrane, numerous cytoplasmic organelles, and form axosomatic synapses with unlabeled terminals. The majority of the labeled processes are dendrites, which contain ribosomes, microtubules, mitochondria, and scattered vesicles. These dendrites are postsynaptic to unlabeled axon terminals and show membrane specializations with other labeled dendrites and perikarya. In contrast to dendrites, peroxidase-labeled profiles clearly distinguished as axons or axon terminals are sparse and never show membrane specializations with other neuronal or nonneuronal structures within the area postrema. Numerous large processes which could be either axons or dendrites are associated with blood vessels and the ventricular surface of the area postrema. With respect to blood vessels, processes are located either in direct apposition to the external glial membrane, or less frequently, within the perivascular space. The ventricular processes are either associated with blood vessels in the subpial space or distributed among the cilia and villi at the anterior margins of the area postrema. The neuronal and nonneuronal associations of the tyrosine hydroxylase-labeled processes are consistent with a receptor or chemosensor function for catecholamines in this circumventricular organ.