The sex chromosomes of the partly sympatric species of gerbils Gerbillus pyramidum and G. gerbillus (Mammalia: Gerbillinae) were investigated by a variety of light- and electron-microscope methods, including DNA replication banding and synaptonemal complex (SC) techniques. The sex-chromosome mechanism of G. pyramidum is of the JXY : XX type, whereas that of G. gerbillus is of the less common (JXYJY2 : XX system. The results include the demonstration that the X chromosomes of both species are compound. One segment is added to the X chromosome of G. pyramidum, leading to an increase in length from the standard 5% to approximately 7.3%, whereas two different extra segments increase the length of the X chromosome of G. gerbillus to approximately 11 % of the length of the haploid genome. In both cases the extra material is autosomal and is also represented in the respective Y chromosomes. Classifying heterochromatin by the variation in staining quality was helpful in elucidating the possible origin of the different chromosome segments, including the pericentromeric regions. Observations on meiotic chromosome pairing and chiasma formation have confirmed the homologies established by band comparisons. The occurrence of chiasmata between the sex chromosomes supports the autosomal origin of the pairing segments. These and other findings have been interpreted in the framework of a multistep evolutionary model. This sequence starts from a hypothetical pair of sex chromosomes, the X element of which amounts to 5% of the haploid genome, and leads through three translocations involving two pairs of autosomes and one pericentric inversion to the most complex situation of this series, manifested in G. gerbillus. The adaptive value, if any, of autosome incorporation into the sex chromosomes repeatedly occurring here is unknown. It is, however, a remarkable fact that in one species, G. gerbillus, the complex sex-chromosome constitution is conserved over vast geographic distances, and in the other, G. pyramidum, the compound X and Y chromosomes withstand change in the face of extreme autosome restructuring.