In situ hybridization of pachytene bivalents of a male Chinese hamster with 125Ilabeled rRNA permitted localization of rDNA. An analysis of the regional distribution of autoradiographic grains from 39 cells showed that rDNA sites were localized in the telomeric regions of chromosome arms 3q, 4q, 5q, 7q, 8p, and 9q. Of these, the sites on 3q, 4q, 5q, and 9q hybridized more frequently and also had significantly more autoradiographic grains than those on 7q and 8p, suggesting that the former four chromosomes had more rDNA than the latter two. The sex chromosomes did not exhibit specific labeling. Silver staining of pachytene chromosomes and sequential silver and quinacrine staining of somatic chromosomes of a second male animal permitted detection of metabolically active ribosomal genes at meiosis and at mitosis. All 12 sites were found to be potentially able to express nucleolus organizing function at meiosis, while expression by individual chromosomes was found to be variable at mitosis in this animal. Chromosomes 3, 4, and 9 showed a high degree of expression (91–97’% of cells with Ag-NORs). Chromosome 8 showed expression to a lesser degree (60% of cells with Ag-NORs), while chromosome 7 showed the weakest expression (29% of cells with Ag-NORs) of all chromosomes that had NORs. Expression by chromosome 5, while high (83% of cells with Ag-NORs), was probably restricted to one homolog. These results permitted two conclusions to be drawn: (1) the amount of rDNA present probably determined the frequency with which its metabolic activity was expressed, and (2) the two animals probably varied with respect to either the amount of rDNA present or its ability to express rDNA function on chromosom