Silver-stained cells from 13 specimens of Akodon arviculoides (2n = 14 and 15) were analyzed for the location and frequency of Ag-NOR’s. They were located terminally on four pairs of chromosomes, 3q, 4q, 5p, and Xp. In these 13 individuals, the number of Ag-NOR’s per cell varied between two and eight, and each chromosome had a characteristic pattern of frequency of NOR’s in each individual. Since in A. arviculoides a chromosomal polymorphism resulting from three autosomal pericentric inversions in chromosomes 2, 3, and 5 and a complex rearrangement involving chromosome 1 exist, it was possible to determine the frequency of Ag-staining in each homologue of these pairs in heterozygous individuals. Both X chromosomes had active NOR’s in the females. An Ag-NOR was also present on the single X chromosome of th