The relationship between the oxidative substrate-turnover-rate and the active Na+-transport was investigated. Our special interest was directed to the possible dependence of the oxidative turnover-rate of one selected substrate from the mode of reduction of the Na+-transport. The following methods were used: 1. Blocking the active Na+-transport by the addition of ouabain to the perfusion medium (0, 5 mg/ml); 2. Reduction of the sodium concentration in the perfusion medium to 100mM substituting NaCl by Tris/HCl; 3. Reduction of the perfusion pressure resulting in a low GFR and a low tubular Na+-load. The perfusion medium contained the following substrates in normal plasma concentrations: Na-palmitate, Na-oleate, Na-propionate, Na-acetate, D-glucose, Na-pyruvate, Na-L-lactate and Na-glutamate. The substrate in question was uniformly 14C-labelled; its amount oxidized per unit time was calculated on the basis of the 14CO2 formed. The following substrates were studied; Na-acetate, Na-pyruvate and Na-L-lactate. There were no significant differences between the results of the three methods described.