Effects of aspartate, glutamate and an inhibitor of the aspartate/glutamate antiporter, diethylpyrocarbonate (DEPC), on uncoupling of the energy transduction processes in rat liver mitochondria have been investigated. It is found that both the antiporter substrates and the antiporter inhibitor operate as recouplers when uncoupling is caused by free fatty acids (FFA). Recoupling consists in (1) partial inhibition of the FFA-stimulated respiration and (2) some increase in the membrane potential. Half-maximal effects are observed at concentrations of glutamate and aspartate close the K(m) values of the antiporter. Recouplings by glutamate (aspartate) and DEPC are not additive. On the other hand, recoupling by any of these compounds and carboxyatractylate or ADP appears to be additive. Uncoupling by dinitrophenol is less sensitive to the recouplers whereas that by FCCP is not sensitive at all. It is concluded that uncoupling by FFA in rat liver mitochondria is mediated not only by the ATP/ADP antiporter but also by the aspartate/glutamate antiporter.