The contribution of molecular slippage of proton pumps, of proton leak and of coupling heterogeneity of mitochondrial population to the well-known non-linear interrelationship between resting state respiration and the protonmotive force is discussed in view of the following experimental findings. (1) After blocking mitochondrial respiration with cyanide, the rate of dissipation of the membrane potential is non-linearly dependent on the actual membrane potential, similarly to the resting state respiration in mitochondria titrated with small amounts of an inhibitor. In contrast, delta pH dissipates proportionally to its actual value. (2) The rate of electron flow from succinate to ferricyanide depends upon the protonmotive force, similarly to the flow from succinate to oxygen. This strongly suggests that the H+/e- stoichiometry in complexes III and IV of the respiratory chain is constant. (3) Mitochondria 'in situ', in permeabilized Ehrlich ascites cells, exhibit the same non-linear flux/force relationship as isolated mitochondria. These results strongly suggest that the non-ohmic characteristics of the inner mitochondrial membrane, with respect to protons driven by the membrane potential but not by the concentration gradient, is the main factor responsible for the nonlinear flux/force relationship in resting state mitochondria.