The non-linear relationship between respiration rate and protonmotive force in isolated mitochondria is explained entirely by delta p-dependent changes in the proton conductance of the mitochondrial inner membrane and is not caused by redox slip in the proton pumps. Mitochondrial proton leak occurs in intact cells and tissues: the futile cycle of proton pumping and proton leak accounts for 26% +/- 7% of the total oxygen consumption rate or 33% +/- 7% of the mitochondrial respiration rate of isolated hepatocytes (mean +/- S.D. for 43 rats); 52% of the oxygen consumption rate of resting perfused muscle and up to 38% of the basal metabolic rate of a rat, suggesting that heat production may be an important function in the proton leak in homeotherms. Together with non-mitochondrial oxygen consumption, it lowers the effective P/O ratio in cells from maximum possible values of 2.33 (palmitate oxidation) or 2.58 (glucose oxidation) to as low as 1.1 in liver or 0.8 in muscle. The effective P/O ratio increases in response to ATP demand; the ability to allow rapid switching of flux from leak to ATP turnover may be an even more important function of the leak reaction than heat production. The mitochondrial proton conductance in isolated mitochondria and in hepatocytes is greatly modulated by thyroid hormones, by phylogeny and by body mass. Usually the reactions of ATP turnover change in parallel so that the coupling ratio is not greatly affected. Changes in proton leak in tissues are brought about in the short term by changes in mitochondrial protonmotive force and in the longer term by changes in the surface area and proton permeability of the mitochondrial inner membrane. Permeability changes are probably caused by changes in the fatty acid composition of the membrane phospholipids.