Using an immunocytochemical method for calretinin (CR) detection, we have earlier described (Morfologiya, 2009 v. 135. No. 3, p. 7-19) the population of previously unknown mono- and bipolar cells with polymorphous spines (PS) covering their cell bodies and processes, in adult mice forebrain structures adjacent to anterior horn of lateral ventricle. CR-positive spiny (CR+PS) cells were negative to GAD67 and were detected in the white matter and in layers V and VI of frontal area of dorsomedial cortex close to the cingulum, in in rostro-dorsal part of the caudate nucleus-putamen complex, anterior olfactory nucleus and in subependymal layer of the dorso-lateral angle of the lateral ventricle. In this work, the distribution of these cells in 7-day-old mice was studied. Comparative topographical analysis of definitive and early CR+PS cells demonstrated that in 7-day-old mice CR+PS cells were absent from the areas of their localization in adult animals - anterior olfactory nucleus, cortical plate and inner portion of neostriatum. Meanwhile, some CR+PS-like cells were detected in 7-day-old mice inside the rostral migratory route, close to neostriatum anterior boundary, along the dorsal border between neostriatum and corpus callosum, subependymal layer of lateral wall of the lateral ventricle, and in the cingulum area. These findings are indicative of the possible postnatal appearance of CR+PS cells. To test this hypothesis, the experiments were conducted in which bromodeoxyuridine (BrdU) was administered to the mice on their postnatal days 2-4 with the subsequent study of the brain sections of these animals sacrificed on their postnatal day 20. Double immunolabeling of these sections for CR and BrdU has detected the presence of CR+PS cells that contained postnatally administered BrdU. These results strongly suggest that, at least, some portion of CR+PS cells have their mitosis postnatally. It may be assumed, that CR+PS cells migrate to the sites of their distribution in adult animals from day 7 till day 20 of postnatal develpment.