1. Comparisons of chromosomes and gene maps of different mammals are yielding a big picture of the evolution of mammalian genome form and function. It has been particularly instructive to compare gene arrangements on the sex chromosomes between the three major groups of mammals. Eutheria (so-called placental mammals). Metatheria (marsupials) and Prototheria (monotremes), which diverged 150 and 170 Myr BP respectively. 2. A region amounting to 3% of the haploid genome is located on the X chromosome in all three groups, implying that this region must have been part of the original X in a common ancestor. This region comprises the long arm of the human X. 3. A region represented by the short arm of the human X is common to the X in all eutherians, but is autosomal in marsupials and monotremes; thus it was not a part of the original X, and must have been acquired by the X early in the eutherian radiation. 4. This recently acquired region was probably translocated to a pseudoautosomal region shared by the eutherian X and Y. Thus it was originally paired and exempt from X chromosome inactivation; stepwise deletion of this region from the Y and recruitment of the newly unpaired region of the X into the inactivation system could account for some of the peculiarities of this region of the human X. 5. The sex-determining gene TDF must lie on the Y in all mammals in which the Y is male determining. The autosomal location of the candidate gene ZFY in marsupials and monotremes eliminates it from consideration. The recently described candidate gene SRY has yet to pass the "marsupial test".