Pyruvate oxidation and swelling in pyruvate solutions by corn (Zea mays) mitochondria were inhibited by alpha-cyano-4-hydroxy-cinnamic acid, an inhibitor of pyruvate transport in animal mitochondria; however, there was no inhibition of pyruvate dehydrogenase activity, and malate and NADH oxidation were not affected. These results suggest the presence of a pyruvate(-)-OH(-) exchange transporter which supplies the mitochondrion with oxidizable substrate. Lactate appears to be transported also, but not dicarboxylate anions or inorganic phosphate. The rate of pyruvate transport was much slower than that of malate, however, and valinomycin was required to elicit appreciable swelling in potassium pyruvate.Malate oxidation contributed significantly to respiration supported by pyruvate plus malate, and malate did not act solely as a "sparker" for pyruvate oxidation. NAD(+)-malic enzyme activity was found in sonicated preparations, and comparison of O(2) consumption with CO(2) released from 1-(14)C-pyruvate indicated that transported malate was being converted to pyruvate, particularly as the malate to pyruvate ratio increased. The results suggest that pyruvate transport becomes limiting under conditions of high energy demand, but that rapid malate transport makes up the difference, supplying pyruvate via malic enzyme and replenishing losses of tricarboxylic acid cycle intermediates.