In the present work we examined the status of nucleolus organizing regions of mitotic chromosomes (NOR) in hybrid cells obtained by fusion of the mouse teratocarcinoma cells PCC4azal and adult mouse spleenocytes upon cultivation of hybrid cells under different conditions. We have shown that extended cultivation of hybrid cells in medium supplemented with HAT (hypoxanthine, aminopterin, thymidine) promotes the maintenance of NO-chromosomes, whereas under nonselective conditions elimination of NO-chromosome occurs. In nonselective medium the number of active, i. e. Ag-positive, NORs has been augmented comparatively to that observed under selective conditions. This observation directly indicates that reprogramming of the parental cell genomes in hybrid cells includes changes in the status of chromosomal NORs. The number of active NORs depends on conditions of hybrid cells culturing and may be changed by either of the two major ways - by elimination of NO-chromosomes (under nonselective conditions) or by inactivation of some NORs, when the general number of NO-chromosomes remains unaltered (under selective conditions).