Leucine and α-ketoisocaproate (α-KIC) were perfused at increasing concentrations into rat brain hippocampus by microdialysis to mimic the conditions of maple syrup urine disease. The effects of elevated leucine or α-KIC on the oxidation of L-[U-¹⁴C]glutamate and L-[U-¹⁴C]glutamine in the brain were determined in the non-anesthetized rat. ¹⁴CO₂ generated by the metabolic oxidation of [^l4C]glutamate and [¹⁴C]glutamine in brain was measured following its diffusion into the eluant during the microdialysis. Leucine and α-KIC exhibited differential effects on ¹⁴CO₂ generation from radioactive glutamate or glutamine. Infusion of 0.5 mM α-KIC increased [^l4C]glutamate oxidation approximately 2-fold; higher concentrations of α-KIC did not further stimulate [¹⁴C]glutamate oxidation. The enhanced oxidation of [¹⁴C]glutamate may be attributed to the function of α-KIC as a nitrogen acceptor from [¹⁴C]glutamate yielding [¹⁴C]α-ketoglutarate, an intermediate of the tricarboxylic acid cycle. [¹⁴C-]glutamine oxidation was not stimulated as much as [¹⁴C-]glutamate oxidation and only increased at 10 mM α-KIC reflecting the extra metabolic step required for its oxidative metabolism. In contrast, leucine had no effect on the oxidation of either [¹⁴C]glutamate or [¹⁴C]glutamine. In maple syrup urine disease elevated α-KIC may play a significant role in altered energy metabolism in brain while leucine may contribute to clinical manifestations of this disease in other ways.