The midgut of the tobacco hornworm (Manduca sexta) is a highly aerobic tissue that is destroyed by programmed cell death during larval-pupal metamorphosis. The death of the epithelium begins after commitment to pupation and soon after commitment there is a decline in the oxygen consumption of isolated midgut mitochondria. In order to assess the role of the electron transport chain in this decline in mitochondrial function, the maximal activities of Complexes I through IV of the respiratory chain were measured in isolated midgut mitochondria. Whereas there were no developmental changes in the activity of Complexes I or III, mitochondria from precommitment larvae did have higher activities of Complexes II and IV (cytochrome c oxidase; COX) compared to those of postcommitment larvae. This finding is consistent with a higher rate of succinate oxidation in mitochondria isolated from precommitment larvae and reveals that the metamorphic decline in mitochondrial respiration is due to the targeted destruction or inactivation of specific sites within the mitochondria rather than the indiscriminate destruction of the organelles. The COX turnover number (e^-/sec·cytochrome ₃) was greatest for the enzyme from precommitment larvae, indicating that during the early stages of metamorphosis there is a change in the enzyme structure and/or its lipid environment. The turnover number of COX in the intact mitochondria (in organello COX) was also lower in postcommitment larvae. In addition to changes in the protein or membrane phospholipids, the metamorphic decline in this rate constant may be a result of the observed loss of endogenous cytochrome c.