Cytoarchitectonics of the lateral tuberal nucleus--a phylogenetically new hypothalamic structure--was investigated using morphometric methods. The most characteristic feature of the human lateral tuberal nucleus is an extremely great variability of its shape and segmentation. The lateral tuberal nucleus consists mostly of several separated neuronal groups; in each brain the arrangement of these groups is different. In macaca the lateral tuberal nucleus is a single elongated neuronal structure much less variable than in man. The human LTN is approximately ten times larger than that in macaca, but the number of neurons is only about four times higher due to a lower neuronal density. The lateral tuberal nucleus both of man and macaca is a homogenous neuronal population with a very low coefficient of variability of cross-section neuronal area (30% and 24% respectively). The size of neuronal bodies in the lateral tuberal nucleus is about 42% smaller in macaca than in man. However, the size of neuronal nuclei is slightly (15%) bigger and the nucleus/cell body area ratio is about twice as high in macaca as that in man (40% and 21% respectively).