The rat and mouse had a common ancestor 15-40 Myr ago, and although substantial chromosomal rearrangements have occurred since they diverged, there is still a high degree of similarity in gene organization in the two genomes. Taking advantage of this similarity, mapping information can be transferred between the two genomes and prediction of positions for hitherto unmapped genes can be made with a high degree of accuracy. In this work, we have put together available information for 916 orthologous rat and mouse gene pairs and, with very few exceptions, all of the gene pairs fell into 52 distinct chromosomal segments (sex chromosomes not included). Most of these segments were confirmed by mouse-on-rat heterologous painting (zoo-FISH) and they were used to make up the backbone of a rat-mouse comparative map. This comparative map was used as a framework for making a rat-mouse prediction map. Predictions for the rat genome were made in two ways. Firstly, the relative position for each orthologous gene pair that cannot be deduced from rat gene data only was suggested from mouse gene data. Secondly, the tentative position in rat of approximately 5100 genes was inferred from the mouse. Thus, this comparative map confers a six-fold increase in the number of gene localization's available for the rat. In addition, the comparative map offers an efficient tool for exchanging genome information between rat and mouse.