Sodium and potassium ion-activated adenosinetriphosphatase (EC number 3.6.1.3) activity, measured as the uptake of 86 rubidium (an analogue of potassium) was determined on peripheral lymphocytes isolated from 20 normotensive obese subjects and 20 normal weight subjects. No difference in the total uptake of 86Rb or in the Na, K-ATPase-dependent uptake was observed in either group. Furthermore, no correlation between the body mass index (BMI) and the Na,K-ATPase-dependent 86Rb uptake was observed. However the Na,K-ATPase mediated 86Rb uptake was always positively correlated with basal blood insulin levels and the insulin sensitivity index. It may be concluded that no lymphocyte dysfunction of Na,K-ATPase was present in our obese patients and that its activity is controlled by insulin in both normal-weight and obese subjects.