Growth of E. coli in the presence of the protonophorous uncoupler pentachlorophenol is shown to strongly enhance levels of cytochrome d, a putative Na(+)-motive oxidase. This effect was found to be arrested by chloramphenicol and stimulated by high Na+ concentration in the growth medium. The induction of cytochrome d takes place in a mutant deficient in the F0F1 ATP-synthase but does not occur in mutants deficient in either of two different components of the Arc system. Similar relationships were revealed when pentachlorophenol was replaced by ferricyanide and phenazine methosulfate, agents oxidizing the respiratory chain. Induction of cytochrome d is also shown to occur in riboflavin-deficient mutants growing in the presence of such low riboflavin concentrations as to be insufficient to maintain a high respiration rate. It is suggested (i) that it is delta mu H+ decrease rather than reduction of the respiratory chain that is the signal for the induction of cytochrome d, and (ii) the Arc system is involved in this type of metabolic regulation.