The complete genomic sequence of the rat APRT gene is described and compared with published mammalian sequences. The rat APRT gene organization is typical of other rodent APRTs with five exons, one large intron of 993 bp, and three smaller introns averaging 145 bp. Because complete sequences for mouse and Chinese hamster APRT are also known, it is possible to compare the evolutionary rates of change in the exons with those of the introns. The latter provide one possible estimate of underlying rates of change. It is shown that the APRT exons have differential rates of evolution in rodents and have had a recent and rapid burst of substitutions within the mouse lineage. Rates of change in the exons do not appear to be strongly correlated with the rates of change in the introns.Key words: APRT, purine salvage enzymes, rates of evolution, rodent.