Menkes disease and Wilson disease are human disorders of copper transport caused by mutations in distinct genes encoding similar copper-transporting P-type ATPases. These genes are expressed in different adult tissues in patterns reflecting disease manifestations. The mouse homologues for the Menkes (MNK) and Wilson (WND) disease genes are the mottled (Atp7a) and toxic milk (Atp7b) genes, respectively. Using RNA in situ hybridization we describe the distribution of mottled and toxic milk transcripts during mouse embryonic development. The mottled gene is expressed in all tissues throughout embryogenesis and is particularly strong in the choroid plexuses of the brain. Mottled expression in the liver is in contrast to the prior observation of absent or very low expression in the adult liver. Expression of the toxic milk gene is significantly more delimited, with early expression in the central nervous system, heart and liver. Later in gestation, toxic milk transcript is clearly seen in the liver, intestine, thymus and respiratory epithelium including nasopharynx, trachea and bronchi. In lung, toxic milk expression is restricted to bronchi, while mottled expression is diffuse. Hepatic expression of both toxic milk and mottled is in the parenchyma, as opposed to blood cells. These results suggest that the mottled gene product functions primarily in the homeostatic maintenance of cell copper levels, while the toxic milk gene product may be specifically involved in the biosynthesis of distinct cuproproteins in different tissues.