Comparative mapping of X-linked genes has progressed rapidly since Ohno's prediction that genes on the X chromosome should be conserved as a syntenic group in all mammals. Although several conserved blocks of homology between human and mouse have been discovered, rearrangements within the X chromosome have also been characterized. More recently, some exceptions to Ohno's law have been reported. We have used fluorescence in situ hybridization (FISH) to map five genes, Gla, G6pd, Hprt, Pgk1 and Xist, to two of the largest conserved segments of X material in five members of the genus Microtus (grey vole) and show that vole X chromosomes demonstrate greater homology to human than to mouse. Cytogenetic analysis indicates a relatively high frequency of rearrangement during vole evolution, although certain blocks of homology appear to be highly conserved in all species studied to date. On this basis we were able to predict the probable location of the rat X inactivation centre (Xic) based solely on high-resolution G-banding. Our prediction was then confirmed by mapping the rat Xist gene by FISH. The possible significance of conserving long-range chromosome structure in the vicinity of the Xic is discussed with respect to the mechanism of X inactivation.