Aerobically grown yeast cells express mitochondrial lactate dehydrogenases that localize to the mitochondrial inner membrane. The D-lactate dehydrogenase is a zinc-flavoprotein with high acceptor specificity for cytochrome c, that catalyzes the oxidation of D-lactate into pyruvate. In this paper, we show that mitochondrial respiratory rate in phosphorylating or non-phosphorylating conditions with D-lactate as substrate is stimulated by carboxylic acids. This stimulation does not affect the yield of oxidative phosphorylation. Furthermore, this stimulation lies at the level of the D-lactate dehydrogenase. It is non-competitive, hyperbolic and its dimension is directly related to the number of carboxylic groups on the activator. The physiological meaning of such a regulation is discussed.