Human inosine monophosphate dehydrogenase activity is the result of the expression of two independent but closely related genes, termed IMPDH type I and type II. We have documented the presence of multiple, processed pseudogenes of type I IMPDH in human and Rhesus monkey genomic DNA, as well as a single functional gene encoding low levels of type I mRNA in human brain, heart, kidney and placenta. Single copy genes for each IMPDH isoenzyme were also found in rat, mouse, dog, cow, and chicken DNA and distinct mRNA species for type I and type II were identified by Northern blots in mouse and hamster RNA. Northern blot analysis of chicken RNA revealed a single mRNA species that hybridized to human IMPDH type I and II probes. These data document the high degree of evolutionary conservation of these two genes among mammals.