Complex study of the structure of islands of Calleja in carnivore brain (cat and dog) was conducted. Using Nissl and Golgi methods, HRP axonal transport and electron microscopy the islands were found to be composed of cells of 3 types varying in size, shape, dendrite spatial distribution and ultrastructure. The majority of cell population in the islands is formed by small granular cells with scarce medium-sized, mitral-like and larger cells among them. The administration of retrograde marker into posterolateral hypothalamus provided the evidence for the presence of cells in the islands of Calleja projecting to this area. These were found to be mitral-like cells. Other cells did not exhibit staining. The resemblance between mitral-like and granular cells of the large islands of Calleja with mitral-like and granular cells of olfactory bulb, respectively, supports our assumption that these islands compose the central sensory nucleus (presumably of the sensory terminal nerve).