By means of the Giemsa staining technique described previously, characteristic banding patterns can be made visible in rat chromosomes. All elements of the karyotype can be distinguished. At the centromeric regions of most chromosomes, striking dark spots can be detected. However, these dark regions are much less prominent than in mouse chromosomes. The X chromosome of Rattus norvegicus comprises 5.31% of the homogametic haploid set. The banding patterns of the X chromosomes in man, mouse, and rat are compared. In metaphase plates of R. norvegicus, an association between autosomes No. 3 and No. 11 is frequently observed. This finding could be interpreted like the analogous phenomenon occurring between the nucleolus organizer chromosomes in human cells.