AbstractGlutaric acidemia type I is an inherited metabolic disorder biochemically characterized by tissue accumulation of predominantly glutaric acid (GA). Affected patients present frontotemporal hypotrophy, as well as caudate and putamen injury following acute encephalopathic crises. Considering that the underlying mechanisms of basal ganglia damage in this disorder are poorly known, in the present study we tested the effects of glutaric acid (0.2–5 mM) on critical enzyme activities of energy metabolism, namely the respiratory chain complexes I–IV, succinate dehydrogenase and creatine kinase in midbrain of developing rats. Glutaric acid significantly inhibited creatine kinase activity (up to 26%) even at the lowest dose used in the assays (0.2 mM). We also observed that CK inhibition was prevented by pre‐incubation of the homogenates with reduced glutathione, suggesting that the inhibitory effect of GA was possibly mediated by oxidation of essential thiol groups of the enzyme. In addition, the activities of the respiratory chain complex I–III and of succinate dehydrogenase were also significantly inhibited by 20 and 30%, respectively, at the highest glutaric acid concentration tested (5 mM). In contrast, complexes II–III and IV activities of the electron transport chain were not affected by the acid. The effect of glutaric acid on the rate of oxygen consumption in intact mitochondria from the rat cerebrum was also investigated. Glutaric acid (1 mM) significantly lowered the respiratory control ratio (state III/state IV) up to 40% in the presence of the respiratory substrates glutamate/malate or succinate. Moreover, state IV respiration linked to NAD and FAD substrates was significantly increased in GA‐treated mitochondria while state III was significantly diminished. The results indicate that the major metabolite accumulating in glutaric acidemia type I moderately compromises brain energy metabolism in vitro.