Supported, in part, by grants from Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil) and DECIT/Ministério da Saúde (MS) through Edital Acidentes e Trauma 2004, CNPq through Edital Universal 2003 and 2006, Fundação Universitáriã José Bonifácio (FUJB, Brazil) through Prêmio Antônio Luiz Vianna 2004 and Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro (FAPERJ, Brazil) through APQ-1 and Fundação Oswaldo Cruz - PAPES IV (Brazil). Drs. Oliveira and Galina are research scholars from CNPq.Objective: Mitochondrial dysfunctions have been associated with the pathogenesis of sepsis. A systematic survey of mitochondrial function in brain tissues during sepsis is lacking. In the present work, we investigate brain mitochondrial function in a septic mouse model. Design: Prospective animal study. Setting: University research laboratory. Subjects: Male Swiss mice, aged 6-8 wks. Interventions: Mice were subjected to cecal ligation and perforation (sepsis group) with saline resuscitation or to sham operation (control group). Measurements and main results: Oxygen consumption was measured polarographically in an oximeter. Brain homogenates from septic animals presented higher oxygen consumption in the absence of adenosine 5'-diphosphate (state 4) compared with control animals. The increase in state 4 respiration in animals in the cecal ligation and perforation group resulted in a drastic decrease in both respiratory control and adenosine 5'-diphosphate/oxygen ratios, indicating a reduction in the oxidative phosphorylation efficiency. Septic animals presented a significant increase in the recovery time of mitochondrial membrane potential on adenosine 5'-diphosphate addition compared with control animals, suggesting a proton leak through the inner mitochondrial membrane. The septic group presented a general reduction in the content of cytochromes. Moreover, the activity of cytochrome c oxidase was specifically and significantly decreased in the brain during sepsis. Hydrogen peroxide generation by brain mitochondria from septic mice did not respond to substrates of electron transport chain or to adenosine 5'-diphosphate, showing that mitochondrial function may be compromised in a critical level in the brain during sepsis. Conclusions: The mitochondrial dysfunctions demonstrated here indicate that uncoupling of oxidative phosphorylation takes place in the brain of septic mice, compromising tissue bioenergetic efficiency.