The effect of 6-benzylaminopurine (BA) was assayed on malate oxidation in mitochondria isolated from fresh and aged potato (Solanum tuberosum L.) slices. Depending on the experimental pH, two pathways for malate oxidation were selected. A pH of 7.7 favored the activity of malate dehydrogenase, which is connected with a rotenone-sensitive NADH dehydrogenase, whereas at pH 6.5 malic enzyme, linked to a rotenone-resistant NADH dehydrogenase, was more active.Experimental results indicate the existence of two sites of inhibition for BA. The first site is common with the site of inhibition of rotenone. The second site is on the classical cyanide-resistant alternative pathway, but is different from the site of salicylhydroxamic acid (SHAM) inhibition, as in succinate oxidation.Moreover, a distinct cyanide-resistant pathway, sensitive to SHAM but resistant to BA, is found to coexist with the well-known alternative pathway which is sensitive to SHAM and BA. This outlet of electrons can accommodate 10% of the total electron flow in mitochondria from fresh slices, and up to 30% in mitochondria from aged slices.