Changes in the activity, in terms of O2 uptake and CO2 output in presence of malate and succinate substrates and of succinic and malic dehydrogenases, diaphorase and cytoehrome-c reductase, of mitochondrial preparations from the peel and pulp of apple tissue, were followed during the development of the respiration climacteric in fruit on the tree. There was a general increase in activity beginning in peel tissue. Oxidative phosphorylation by the mitochondrial preparations was maintained at a high level throughout the period. At the same time a threefold increase in malic enzyme (M.E.) and carboxylase activity was observed. It is suggested that these observations show that the climacteric rise in apple fruits attached to the tree is initiated by an increase in mitochondrial activity and develops through the joint action of increased activity of M.E. and carboxylase.