Data obtained by using ultrastructural and morphometric approaches revealed three types of neurons in the cat visual claustrum. The most numerous were medium-sized and large ones. They formed 3/4 of the cell population. The ultrastructural properties of those cell types were largely similar. Their cell bodies were oval, round, fusiform or triangular and contained more or less indented nuclear envelope. The cytoplasm of those cells was characterized by a high concentration of subcellular organelles and particularly rough endoplasmic reticulum. The characteristic feature of those cells was a low nucleus/cell body area ratio (47 ± 1% and 43 ± 1%, respectively). The proximal dendrites of medium-sized cells were usually wide at the base, relatively short and tapering, whereas, those arising in the large cells were often thick and had a short tapering base. The neurons described above stained by Golgi impregnation method shoved spines on their distal dendrites both under the light and electron microscopy. The retrograde axonal transport of HRP and WGA-HRP following injections into the visual cortex confirm that they are mainly projection cells, which form the ascending limb of the claustrocortical loop. The third type of neurons formed a less numerous group of small cells, which differed from the larger ones in various respects. They possessed the large nuclei with deeply indented nuclear envelope and comparatively a thin layer of cytoplasm poor in subcellular organells among which free ribosomes and mitochondria were common. The nucleus/cells body area ratio high (59 ± 2%). In Golgi preparations their dendrites did not show spines. The dendrites originating from that type of neurons were thin, long and did not possess a wide tapering base. They are mainly claustral intrinsic neurons.