The mosquito Aedes aegypti is the vector of several illnesses, such as dengue and yellow fever, and females ingest huge amounts of blood to meet their energy requirements. Blood digestion poses a challenge to this organism since heme release is associated to many deleterious effects. An hypothesis raised by our group suggest that hematophagous organisms shift their energy metabolism from aerobic to fermentative during blood digestion as a preventive antioxidant defence, avoiding oxygen utilization and reactive oxygen species generation in mitochondria. Little is known about mitochondrial physiology in insects and this led us to investigate the respiratory functions of mitochondria isolated from blood-fed (BF) and non blood-fed (NBF) adult A. aegypti females. The state 3 respiration rate using complex I substrates (pyruvate-proline), was reduced in mitochondria from BF, whereas the oligomycin-induced state 4 respiration was unchanged between BF and NBF. Blood feeding caused a drastic reduction of the respiratory control ratio (RCR) (2.8) compared to NBF (5.8) due to interference in the state 3 respiration. Using glycerol-3 phosphate (G3P) as substrate, the respiratory rates were much smaller than using pyruvate/proline. Moreover, BF unexpectedly increased both state 3 and state 4 respirations compared to NBF, although the RCR of NBF was 2.7 and BF 1.0, indicating a mild uncoupling promoted by blood feeding in G3P induced respiration. Classical electron transport chain inhibtors, such as rotenone, antimicyn A, azide and KCN, blocked the oxygen consumption induced by both complex I and G3P substrates, whereas the uncoupler FCCP, promoted oxygen consumption. Cytochrome c oxidase activity (Cox) was assessed by two different methods and both showed that in BF mosquitoes Cox activity was about half of that found in NBF, indicating that blood-meal causes a impairment in the electron transport chain at the complex IV level, affecting the RCR in these mosquitoes. Measurements of membrane potential (DYm) in mitochondria from NBF revealed that pyruvate/proline induced a hyperpolarization that is partially dissipated by ADP, rotenone or FCCP addition. However, when G3P is used, higher concentrations of this substrate are needed to generate comparable levels of DYm, indicating that G3P respiration contributes less to DYm, reflecting in the lower RCR when this substrate is used. This is consistent with the observation that cytochrome c reduction induced by G3P in submitochondrial particles is about half of the values obtained when NADH was used as an electron donor. Taken together these results indicate that A. aegypti mitochondria complex I substrates were more efficient electron donors than G3P-dehydrogenase substrates and that the capacity to synthesize ATP in mitochondria from BF mosquitoes is diminished.