The neurohypophysis of the South American lungfish Lepidosiren paradoxa has been studied with light and electron microscopy, including the Falck-Hillarp technique for catecholamines. The pars nervosa hypophyseos is a well-marked, dorsally located subdivision of the pituitary gland composed of lobes or follicles, each one constituted of a central core of ependymal cells, a subependymal hilar region made up of nerve fibers and a peripheric palisade zone of nerve endings which contact capillary vessels. Four types of neurosecretory axons can be distinguished under the electron microscope. Type I, the most common, contains spherical elementary granules of high electron density, 1500–1800 A in diameter. The scarce type II axons contain irregularly-shaped elementary granules. Type III contains only small clear vesicles, 400–600 A in diameter. Type IV, mostly present in regions of the gland contacting the pars intermedia, contain large granulated vesicles, 900–1000 A in diameter. The Falck-Hillarp technique revealed axons with a positive reaction for catecholamines at sites corresponding approximately to the location of type IV of the electron microscope. Ependymal cells are of large size, linking the cerebrospinal fluid, the nerve endings and the blood vessels. A conspicuous membrane-bound, spherical dense material, 1400–2000 A in diameter, is observed in both the apical and vascular processes of these cells. The ependymal processes which traverse the hilar and palisade regions contain structures resembling degenerated neurosecretory axons. These results are discussed in relation with the currently available information on the comparative anatomy of the pars nervosa. The possible functional significance of ependymal cells and of each type of axon are also discussed. This study was aided by the following grants: NIH NS 06953 to Prof. De Robertis, Consejo Nacional de Investigaciones Cientificas y Tecnicas to Prof. Zambrano, Comision de Investigaciones Cientificas de la Provincia de Buenos Aires and Comision de Investigaciones Cientificas de la Universidad Nacional de la Plata: to Prof. Iturriza. The authors are indebted to Prof. De Robertis for his generosity in granting us his laboratory facilities, and to Dr. F. J. J. Risso and Mr. A. Fernandez (Resistencia, Chaco) who provided the specimens used in this study. The able microtechnical assistance of Miss L. Riboldazzi and Mrs. R. Rana and the photographic work of Mr. A. Saenz are much appreciated.