The copper content of various organs of `brindled' female heterozygotes and male mice affected by this X-linked mutation are documented at the last day of intrauterine development, at 1 day after birth and at 11 days of age. The findings indicate defective placental transfer of copper in utero, and an even more marked defect in intestinal absorption of copper after birth. In addition there is an abnormal distribution of copper among the tissues of the body once it is absorbed. The mutation produces abnormal accumulation of copper in kidney, in gut mucosa and in testis, whereas liver, brain, plasma and most other organs show diminished copper concentrations. The intestinal malabsorption of copper is accompanied by accumulation of abnormal amounts of the metal in the intestinal-mucosa cells. Copper concentrations in both mucosa and luminal contents rise progressively from duodenum to ileum. Defective upper-intestinal absorption, consequent progressive increase in luminal copper concentration and pinocytosis in the ileum would seem to explain the findings. Radioisotopic studies eliminated the possibility of excessive excretion of copper in bile or across the intestinal mucosa. Detailed comparison with findings in humans with Menkes' syndrome is difficult because of the different stages of development at which the studies have been performed, but the results seem in general to conform very satisfactorily. Those differences seen are probably explicable by known species differences. All the findings are in accord with a hypothesis that the basic defect involves accumulation and retention of copper in the cells of affected tissues such as kidney, gut mucosa and placenta.