The relationship between rate of ATP synthesis and transmembrane electrochemical proton gradient has been determined in rat liver mitochondria oxidizing succinate, using the respiratory inhibitor malonate or the uncoupler FCCP to decrease delta microH progressively. As previously reported [(1982) Eur. J. Biochem. 126, 443-451] two different relationships are obtained depending on the method used. Evidence is presented that this result is not due to underestimation of the delta microH maintained by fast-respiring mitochondria, as recently suggested [(1985) FEBS Lett. 181, 323-327].