<p>3-Hydroxyisobutyric acid dehydrogenase (HIBADH) deficiency is biochemically characterized by tissue accumulation of L-3-hydroxyisobutyric acid (3HIBA). Some patients manifest neurologic symptoms and cerebral magnetic resonance imaging abnormalities whose pathophysiology is unknown. We evaluated the in vitro and in vivo effects of 3HIBA on important parameters of redox homeostasis and oxidative phosphorylation in cerebral cortex of developing rats. The in vitro studies revealed that 3HIBA significantly increased nitrite and nitrate levels and decreased reduced glutathione concentrations (GSH), glutathione peroxidase (GPx) and glutathione reductase activities (GR). 3HIBA also inhibited the activities of complex IV and moderately complex II-III of the respiratory chain, as well as ATP production. Noteworthy, the nitric oxide synthase inhibitor L-N-Nitro-L-arginine methyl ester prevented the augmented levels of nitrate and nitrite, the inhibition of GPx and complex IV activities, and the reduction of GSH and ATP levels caused by 3HIBA, supporting an important role of nitrosative stress in the disruption of redox homeostasis and oxidative phosphorylation. These biochemical markers were not altered by 3-hydroxybutyric acid, the isomer of 3HIBA, indicating a selective property of 3HIBA. Furthermore, intracerebroventricular administration of 3HIBA induced augmented cortical levels of nitrite and nitrate and decreased of GSH concentrations, GPx and complex IV activities. It also increased the mRNA expression of the antioxidant enzymes GPx1, SOD2 and GR, as well as of the oxidative stress modulator NRF2 and of the enzyme iNOS. The present data provide novel evidence of 3HIBA neurotoxicity, bringing new insights on the role of this organic acid in HIBADH deficiency neuropathology.</p>