Xanthine dehydrogenase (XDH), aldehyde oxidase and pyridoxal oxidase are discussed together in this chapter because they share certain features of genetic control, and they seem to be related in a number of ways. These three enzymes and at least six genetic loci, some with multiple alleles, are involved in one of the most complex and interesting gene-enzyme systems yet studied in any eukaryote. Reviews of the early work on this system have appeared elsewhere (Glassman, 1965; Glassman et al., 1968). A more recent discussion may be found in Ursprung (1973).