A comparative study using optical and electron transmission and scanning microscopes was carried out to determine the cells that comprise the wall of the third ventricle of the rabbit (Oryctolagus cuniculus). Three cell types have been distinguished: type I cells are the most numerous and may present a flat, cubical or cylindrical aspect, with cilia and microvilli at their apical end. Type II cells have an irregular elongated morphology, lack cilia on their luminal end and show functional characteristics of secretory cells. Type III cells are also elongated, and bear irregular protrusions at the apical end. A long process at the basal end of these cells goes deep into the neuropil and becomes related with neurons or blood vessels; these correspond to tanycytes.