The time-course of some alterations produced in erythrocytes during the onset of CCl4-induced liver cirrhosis was studied in rats. Erythrocyte membranes were isolated to measure Na+, K+ and Ca+2-ATPase activities. Membrane lipid composition was determined to calculate the cholesterol/phospholipid ratio and serum samples were used to measure lipoperoxidation. The results demonstrated that as CCl4 treatment progressed, serum lipoperoxidation and membrane cholesterol/phospholipid ratio increased while ATPase activities decreased. ATPase activities in red blood cells of cirrhotic rats were 50% below normal values but those determined in cells of animals treated simultaneously with CCl4 + silymarin were significantly improved. Silymarin co-treatment also preserved the normal cholesterol/phospholipid ratio in the membranes. Our results suggest that the measure of ATPase activities in erythrocytes membranes could be a simple, safe and useful early marker of liver damage and also valuable to test the effectiveness of a given drug therapy.