Tanycytes with foot processes contacting capillary basal membranes were identified in the rat medial habenular nucleus. They constitute a relatively small but constant population of cells among the conventional ependymal cells. In contrast to tanycytes in most circumventricular organs, habenular tanycytes possess cilia. Superimposed upon the cells are nerve fibers belonging to the serotonergic supraependymal axon plexus. Their ultrastructure differs in many respects from that of hypothalamic tanycytes.