The portion of Na-K-ATPase activity on oxygen consumption was determined in suspensions of rat proximal tubules by monitoring oxygen consumption (QO2) under different metabolic states: in the presence or absence of succinate or lactate (10 mmol/l) and increasing concentrations of ouabain (0.4; 0.8; 1.2 and 1.6 mmol/l). In the metabolic states tested, the ouabain induced decrease of QO2 was identical, which implies a fixed rate between Na-K-ATPase activity and QO2. On the basis of these results, Ki and maximal inhibition rate were determined by Lineweaver-Burk and Eadie-Hofstee plots Ki was 0.67 mmol/l ouabain and the maximal inhibition of QO2 was 77%. This corresponds to an absolute decrease of QO2 of 1,630 mumol O2 . h-1 . g-1 protein. Since this value represents the real portion of Na-K-ATPase on QO2, the activity of Na-K-ATPase can be calculated, yielding an activity of 163 mumol ATP . min-1 . g-1 tubule protein.