Potato tuber mitochondria oxidizing malate respond to NAD' addition with increased oxidation rates, whereas mung bean hypocotyl mitochondria do not. This is traced to a low endogenous content of NAD' in potato mitochondria, which prove to take up added NAD'. This Macrae and Moorhouse (15). The presence of these two enzymes in the matrix space (7) causes pyruvate and oxaloacetate to accumulate in the medium during malate oxidation. The rates and products of malate oxidation by plant mitochondria vary in response to changes in the pH of the incubation medium (16). In addition, the oxidation of malate which is coupled to three sites of ATP formation is stimulated under certain conditions by NAD+ (6,9,20).With the aim of further clarifying the mechanisms of malate oxidation in intact plant mitochondria this report details the effect of NAD+ on malate oxidation in potato tuber and mung bean hypocotyl mitochondria. Some of these results have been presented elsewhere (17).MATERIALS AND METHODS Preparation of Mitochondria. Mitochondria from potato (Solanum tuberosum L.) tubers and etiolated mung bean (Vigna radiata L.) hypocotyls cut from bean seedlings grown for 5 days in the dark at 26 C and 60%o RH were prepared and purified by methods previously described (10). All operations were carried out at 0-4 C. Following purification, the mitochondria appeared to be virtually free from extramitochondrial contamination and had a high degree of membrane intactness as judged by electron microscopy and by low activities of the inner membrane and ' Supported in part by a grant from the Centre National de la Recherche Scientifique (ERA 847: Interactions Plastes-cytoplasme-mitochondries).