The embryos with trisomy for chromosome 19 were obtained in the crosses of mice heterozygous by two different Robertsonian translocations. The development of these embryos was markedly delayed, in some of them abnormalities of the cranial region of nervous tube and the eyes were observed. The rate of 3H-uridine incorporation into 28S and 18S rRNA in the trisomic embryos both in the in vivo and in vitro experiments was decreased in 1.3-1.4 times, on the average, as compared with th control embryos. The concentration of cytoplasmic RNA in the tissue of trisomic embryos suffered no significant changes. The absence of active nucleolus-forming regions in chromosome 19 upon translocation Rb(5; 19)1Wh and the preservation of nucleolus-forming function of chromosome 19 involved in the centric fusion RB(9;19)163H were demonstrated by means of specific staining with silver nitrate. Possible causes of developmental delay of the trisomic embryos and of absence of the dose effect of ribosomal genes in them are discussed.