A disease, known as ovine white liver disease (OWLD) was experimentally reproduced in lambs by feeding a diet depleted of cobalt. At necropsy, affected animals had pale, swollen, friable fatty livers, and showed marked accumulation of lipofuscin. Control animals, fed the same diet to which adequate amounts of cobalt had been added, were clinically normal. In animals with OWLD, liver triglyceride and free fatty acid concentrations were increased. A decrease in the ratio of phosphatidyl choline to phosphatidyl ethanolamine in the liver may result in a reduced ability to export triglycerides as very low density lipoprotein. This may cause the lipid accumulation characteristic of OWLD. Lipofuscin accumulation, another feature of OWLD, is a consequence of lipid peroxidation. Evidence for a peroxidative challenge was provided by the finding of reduced concentrations of alpha-tocopherol, elevated concentrations of induced 4-hydroxynonenal, and decreased amounts of the most readily peroxidizable fatty acids in the liver of animals with OWLD, by comparison with controls. The initiator of the peroxidative challenge is unknown, but may be related to the finding of increased concentrations of homocysteine in the plasma of animals with OWLD.