Rat liver mitochondria are able to temporarily lower the steady-state concentration of external Ca2+ after having accumulated a pulse of added Ca2+. This has been attributed to inhibition of a putative delta psi-modulated efflux pathway [Bernardi, P. (1984) Biochim. Biophys. Acta 766, 277-282]. On the other hand, the rebounding could be due to stimulation of the uniporter by Ca2+ [Kröner, H. (1987) Biol. Chem. Hoppe-Seyler 369, 149-155]. By measuring unidirectional Ca2+ fluxes, it was found that the uniporter was stimulated during the rebounding peak both under Bernardi's and Kröner's conditions, while no effects on the efflux could be demonstrated. The rate of unidirectional efflux of Ca2+ was not affected by inhibition of the uniporter. It appears likely that the rebounding is due to stimulation of the uniporter rather than inhibition of efflux.