The macular mouse is an X-linked recessive inherited mutant and is considered to be a model for human congenital copper deficiency, Menkes' kinky hair disease. The activity of urate oxidase, which has been believed to be a copper enzyme, and copper content in the liver of the mutant mouse were determined. The oxidase activity was maintained at normal level even though there was very low level of copper present in the liver through days 7 to 14. Copper administration increased the copper content in the liver to the normal level, but did not affect the oxidase activity.