Cerebrospinal fluid (CSF)-contacting neurons were studied by means of electron microscopy in the spinal cord and/or terminal ventricle of the ray, Raja clavata (Elasmobranchii), the opossum, Didelphis virginiana (Marsupialia), the mouse, Mus musculus, and the guinea pig, Cavia cobaya (Rodentia). Dendrites of the CSF-contacting neurons in the spinal cord of the ray penetrate the ependyma of the central canal and form terminals bearing stereocilia. Axons apparently belonging to these neuronal perikarya terminate on the basal lamina of the spinal cord near the fila of the radix ventralis. In the opossum, a representative of metatherian mammals, the dendritic terminals of the CSF-contacting neurons resemble those of the phylogenetically ancient lower vertebrates and are endowed with many stereocilia. In such eutherian mammals as the mouse and the guinea pig, the corresponding stereocilia are usually less developed. There are numerous CSF-contacting neurons in the wall of the terminal ventricle of the mouse. Since the dendritic terminals of the spinal CSF-contacting neurons resemble those of known sensory cells and the axon terminals on the basal lamina resemble ultrastructurally neurosecretory endings, we suppose that the former are receptive to stimuli exerted by the internal (ventricular) CSF and capable of translating them into a neurosecretory output directed toward the external (subarachnoid) CSF. With their periradicular terminations the axons of the CSF-contacting neurons establish an extended, special site for neurosecretory release along the ventrolateral sulcus of the ray spinal cord.