To assess the pathogenetic mechanisms involved in the accumulation of copper in primary biliary cirrhosis (PBC), the kinetics of 64Copper (64Cu) were studied in 6 healthy volunteers, 3 patients with PBC (Stages I to III) and a normal liver copper, and 7 patients with PBC (Stages II to IV) and a high liver copper. The kinetics of oral and i.v. administered 64Cu in patients with a normal liver copper were similar to those in controls. In patients with a high liver copper, the initial disappearance rate from the plasma of i.v. administered 64Cu was more rapid, the disappearance rate from the liver of 64Cu taken up by this organ was slower, and the fecal excretion of 64Cu less as compared with the two other groups of subjects. In addition, the fecal excretion of orally administered 64Cu was less in these patients. Computation of the rate of absorption of orally administered 64Cu revealed no differences between the three groups. Two of the patients with a high liver copper were decopperized by treatment with D-penicillamine. In these patients, the disappearance rate of i.v. administered 64Cu from the liver and the secondary reappearance rate of 64Cu into plasma became normal; however, the fecal excretion of 64Cu remained low. It is concluded that there is no pathognomonic alteration of copper metabolism in PBC, but that in some patients, copper accumulates in the liver due to decreased biliary excretion and increased clearance from the plasma, and that dilution of the tracer dose a large amount of stable copper is responsible for all abnormalities of 64Cu kinetics encountered in PBC, except low fecal excretion, which is caused by impairment of biliary excretion.