When leaves of Bryophyllum tubiflorum were cut into transverse sections, and held at 20 C in the dark, the capacity to accumulate organic acid decreased with decreasing section thickness. In addition, the rate of respiration increased with decreasing section thickness and was unaffected by changes in O(2) concentration above 5% or by the presence (1%) of CO(2). It was concluded that O(2) ventilation is not a controlling factor in respiration. Malonate (0.1 m) and fluoroacetate (0.01 m) restored the capacity of sectioned leaves to accumulate acid to normal levels and depressed respiration in 1-millimeter sections. Acid accumulation in 8-millimeter sections remained essentially constant at 20, 15, and 10 C, and was equal to that in unsectioned leaves, but accumulation in 2-millimeter sections rose to normal levels as the temperature fell to 10 C. Twenty-three additional metabolic inhibitors (none specific to the tricarboxylic acid cycle) were screened, and none promoted acid accumulation in sectioned leaves at 20 C. The results suggest that sectioning stimulates a respiratory sequence which includes the tricarboxylic acid cycle. This sequence in turn competes with the synthesis or accumulation of malic acid.