In this paper we examine the non-linearity of the relationship between the proton electrochemical gradient across the mitochondrial inner membrane (delta p) and oxygen consumption of non-phosphorylating mitochondria in situ in hepatocytes. Models proposing to explain the non-linear relationship were tested experimentally. It was shown that the mitochondrial proton conductance and the number of protons pumped to the cytosolic side of the mitochondrial inner membrane by the electron transport complexes per oxygen atom consumed (H+/O ratio) are independent of electron transport rate in mitochondria in isolated hepatocytes. The non-linearity of the plot of delta p against the non-phosphorylating oxygen consumption is due to either a potential-dependent slippage of the proton pumps of the mitochondrial inner membrane and/or a potential-dependent leakage of protons back across the mitochondrial inner membrane.