Scanning electron microscopic observations reveal cerebrospinal fluid contacting and supraependymal cell bodies of the mesencephalic trigeminal (Mes V) nucleus in adult specimens of mako and blue sharks, but not in very young specimens of the smooth dogfish shark. Possible functional roles are discussed in terms of participation of cerebrospinal fluid contacting Mes V cells in a neuroendocrine regulatory system. Comparisons are made with light microscopic observations of the relationship of Mes V cells with the ventricular wall. General features of the scanning electron microscopic appearance of the roof of the ventricle of the optic tectum in sharks are described and compared to similar general features found in the third ventricle of a variety of vertebrates.