A nucleus of catecholamine-containing cells bordering the preoptic recess of the toad hypothalamus has been studied by both fluorescence histochemical and electron microscopic methods. The perikarya of these cells form one to three rows immediately subjacent to the ependyma. They send brightly fluorescent apical processes between the ependymal cells to the ventricular surface, and also give rise to long basal processes, the proximal portions of which are also fluorescent. These cells contain two distinctive constitutents: juxtanuclear bundles of tightly packed filaments, the members of which are separated from one another by only approximately 100 A, and large numbers of dense-cored vesicles (400-2200 A in diameter), which appear to arise from an agranular tubular reticulum distinct from the Golgi apparatus. Axons containing either clear vesicles alone or clear and dense-cored vesicles form synapses on the subependymal cells, but no evidence has been found that the subependymal cells themselves form presynaptic contacts, or that axons originate from them. The cytological characteristics of these catecholamine-containing cells, plus the fact that they border directly on the cerebrospinal fluid, suggest that they may be more closely related to peripheral chromaffin cells than to the other cell types intrinsic to the central nervous system, and the name "encephalo-chromaffin cells" is therefore proposed for them. The possible functions of such cells in the central nervous system are discussed.