The ventriculus mesencephali of golden hamsters protrudes, in the form of a short channel, from the upper wall of the aquaeductus Sylvii in dorsal direction, between the third and fourth ventricle. It is lined with ependymal cells of cubic to cylindrical forms. Some of the ependymal cells possess basal processes. On the cell surface, there are cilia, microvilli and protoplasmatic protrusions. The cells contain relatively few endoplasmatic reticulum, a poorly developed Golgi apparatus, free and bound ribosomes and critiform mitochondria. The cell nuclei have round to oval shapes. Supraependymally, there are homogenous globules and intraventricular nerve fibres. The morphological characteristics of ependyma in the region of ventriculus mesencephali, as well as the close contact of the ependymal cells with capillaries, allow to suppose an active participation of these cells in the exchange of various substances also in this part of the brain ventricle system.