A study on the Na+-K+ ATPase activity of erythrocyte membranes was carried out in 34 NIDDM patients without complications and in 20 age-matched normal control subjects. The Na+-K + ATPase activity was decreased in the NIDDM patients as compared with that of the normal subjects. There was a positive correlation between the Na+-K+ ATPase and the serum insulin peak level as well as the area under curves of insulin releasing test. There was no correlation between the Na+-K+ATP ase and the fasting blood glucose or glycohemoglobin level. The mean value of erythrocyte su-peroxide dismutase (SOD) was decreased in the NIDDM patients as compared with that of normal subjects. But the Na+-K+ ATPase activity was not correlated with SOD. The result suggests that Na+-K+ATPase activity in the NIDDM patients is regulated by the serum insulin level. Insulin treatment would be useful for these patients to increase the activity of Na+-K+ATPase and prevent chronic complications.