Corriedale ewes were exposed to ration and a pasture containing a high level of copper to study the relationship between peroxidation of lipid membrane and copper-induced hemolysis. We measured phospholipids, cholesterol, and protein from red blood cell (RBC) ghosts, and lipid peroxidation. A decrease in total phospholipids in the erythrocyte membranes and an increase in lipid peroxide levels of erythrocytes was observed in blood samples from animals that ingested large amount of copper. The cholesterol concentration was not altered in RBC. Additionally, depressed phospholipid/protein and phospholipid/cholesterol ratios were found. These data suggest that elevated copper levels have a deleterious effect on membrane integrity through the peroxidation of lipids. It is reasonable to postulate that these alterations were a possible cause of hemolysis.