Having applied the picric acid-bromphenol blue (BPB) and the phosphomolybdic acid-benzidine (PMA-B) methods – the first method detecting the total histone content, the second one only free histones – we have found that cell nuclei of the rat embryos permanently contain histones, nevertheless, in such a pattern that the free histone appears only at a well-defined stage of the embryogenesis. Strong PMA-B reaction is observable in the nuclei of some mesenchymal cells and in the reorganizing somites’ cell nuclei of the 13-day-old embryo as well as in the hemopoietic elements of the 15- and 17-day old embryos’ liver. Within nuclei of the red blood cells of the 13-day-old embryo an extremely intense reaction is seen which then disappears till the 15th day, while the BPB reaction shows a gradual intensification up to the neonatal age. The experiments support the former model of one of the authors for a histone-nonhistone control of onto