Erythrocyte membrane cholesterol, phospholipid, basic Ca2+. Mg2+--ATPase activity and the reaction of the enzyme to membrane-free hemolysate solution in vitro were measured in 32 untreated hyperthyroid patients and compared with the results in 30 age matched normal subjects. Erythrocyte membrane basic Ca2+. Mg2+--ATPase activity is found to be significantly higher in hyperthyroidism and has a significantly positive correlation both to the serum T3 Level and pulse pressure. RBC membrane Ca2+. Mg2(+)-ATPase reaction to membrane-free hemolysate solution is weaker than that in the controls. The amount of red blood cell membrane cholesterol is significantly increased and serum cholesterol is decreased in hyperthyroidism. No significant correlation between the enzyme activity and the amount of the erythrocyte lipids were found.