Ca2+ efflux from rat liver mitochondria can occur when endogenous nicotinamide nucleotides are oxidized. It is suggested that nicotinamide nucleotide induced by acetoacetate sensitizes the mitochondria to damaage resulting from the accumulation of Ca2+ in the presence of Pi. Thus, acetoacetate-induced Ca2+ efflux is associated with a loss of respiratory control. Both the effluxes induced by acetoacetate and by high Ca2+ accumulation are prevented by ATP plus oligomycin, although these agents do not prevent the endoagenous nicotinamide nucleotides from becoming oxidized on addition of acetoacetate. Acetoacetate addition only results in Ca2+ release if the Ca2+ and Pi concentration are above a critical value. The acetoacetate-induced Ca2+ effflux is exactly paralled by the virtually complete collapse of the membrane potential. The presence of acetoacetate decreases the concentration of total Ca2+ necessary to induced mitochondrial damage by about 130 nmol of Ca2+/mg of protein. It is concluded that acetoacetate-induced efflux occurs by reversal of the Ca2+ uniporter after the collapse of the membrane potential.