It was shown by light and electron 3H-thymidine autoradiography that in the zones of endochondral osteogenesis in a tubular bone the little-differentiated perivascular cells are characterized by the most intensive proliferation among the DNA-synthesizing cells of the stroma. A comparison of the ultrastructural features and the distribution of labelled cells, their topographic interrelations at different times following the isotope injection suggests that during histogenesis in the endochondral centres the little-differentiated perivascular cells become spatially separated and are involved in the formation of the stromal cellular elements (in particular, fibroblasts, reticular cells, osteoblasts). In the developing and mature stroma, as well as in the endost, the little-differentiated forms of the population are preserved; the number of DNA-synthesizing cells among them decreases with age. It is suggested that the population includes stromal stem cells.