The effect of various Krebs cycle acids on the respiration of disks of apple peel at various stages of maturity was measured in a Warburg respirometer. Peel tissue from apples at the pre-climacteric and early post-climacteric stages apparently contain sufficient of the Krebs cycle acids used, with the exception of succinate, to maintain oxidative processes at a maximum. The addition of malate causes a large increase in the CO2-output of peel from post-climacteric and senescent fruit but not from pre-climacteric fruit, and a close correlation exists between the climacteric and this decarboxylation of malate. The decarboxylation of malate does not affect the rate of O2-uptake of peel tissue. The possible part played by the decarboxylation of malate in the increased CO2-output at the climacteric is discussed. Added pyruvate is decarboxylated by the tissue at all stages of storage life. The decarboxylation of added malate is an aerobic fermentation, resulting in the quantitative production of acetaldehyde. Although the presence of oxygen is necessary, the rate of O2-uptake is not affected by the reaction. Pyruvate decar boxylation does not require the presence of oxygen. The O2-uptake of peel from senescent apples can be stimulated by addition of malate, succinate, and a-ketoglutarate. No evidence was obtained, however, of oxidation of fumarate, citrate, or pyruvate. The addition of malate to senescent tissue restores the lower endogenous rate of O2-uptake to that of early post climacteric tissue. Succinate and fumarate are toxic to peel tissue at concentration above 0.02M.