The prenatal patterns of calretinin immunoreactivity were studied in the olfactory systems of Tupaia belangeri. We investigated the peripheral and primary central parts of the vomeronasal system and of the main olfactory system from the 19th to the 43rd (last) day of gestation and compared the findings with the known calretinin immunoreactivity patterns in adult T. belangeri and the published data on other mammals. The onset of calretinin immunoreactivity was noted in the main olfactory system on the 23rd day of gestation and, in the vomeronasal system, on the 25th day of gestation: single precursors of receptor cells with calretinin immunoreactive perikarya and processes were observed in both epithelia. Their neuronal identity was proven by olfactory marker protein immunoreactivity. On the 42nd day of gestation, almost all receptor cells and nerve fibers, many interneurons and projecting cells were calretinin immunoreactive in the main olfactory and in the vomeronasal systems. In contrast to the intensive calretinin labeling previously observed in virtually all vomeronasal epithelial cells of adult T. belangeri, among developing receptor cells a population of intensively labeled, basally located perikarya was distinguishable from a population of less intensively stained, more apically located ones. In the main olfactory epithelium of fetal T. belangeri, calretinin immunoreactive receptor cells occurred in the middle layers. Whereas in the vomeronasal sensory epithelium differently reacting layers of receptor cells might represent the two known subfamilies of receptor cells, in the main olfactory epithelium the differing calretinin expression in the layers of the epithelium, most probably, did not reflect known subfamilies of odour receptor cells. Transiently, ectopic calretinin immunoreactive receptor cells were observed in the future non-sensory epithelium of the vomeronasal organ.