Mitochondria isolated from Kalanchoë daigremontiana, a Crassulacean acid metabolism plant, decarboxylate added malate to pyruvate at rates of up to 100 micromoles per hour per milligram original chlorophyll in the presence of ADP. Omitting ADP reduces this rate by approximately 50%. Antimycin A inhibits malate decarboxylation and this inhibition could be relieved by addition of aspartate and alpha-ketoglutarate to the mitochondria. Increasing the pH of the external medium inhibited malate decarboxylation; a dramatic decrease in pyruvate production was observed between pH 7.2 and pH 7.4. It is suggested that cytoplasmic pH changes may regulate the contribution of mitochondria to malate decarboxylation in the light in vivo.