1. Mitochondria prepared from young Arum spadix oxidise succinic and α-ketoglutaric acids at about 300μl. O2/hr./g. wet wt. but as the spadix ages the activity of the mitochondria increases more than tenfold. The increase results from a small rise in the quantity of mitochondrial nitrogen brought down in the centrifuge and a larger rise in oxidase activity per mg. mitochondrial nitrogen. 2. The rate of respiration of spadix slices has been measured under conditions in which it is not limited by slow diffusion of oxygen. Slices of young spadix respire at about 1,000–2,000 μl. O2/hr/g. but as the inflorescence opens the rate soars to 20,000μl. O2/hr/g. and the spadix becomes warm. 3. Cytochrome oxidase was assayed in mitochondria treated with digitonin. There is at all times enough cytochrome oxidase to account for the rates of oxidation of succinate and α-ketoglutarate; and enough to account for the relatively slow respiration of the young spadix slices but not for the fastest respiration of the mature spadix. 4. The respiration of spadix slices is found, in contradiction to an earlier report, to be sensitive to malonate.