The effects of amino acid imbalance on brain glutamic acid metabolism were studied in male albino rats to note how it affects the glutamic acid system of brain. The amino acid imbalance was induced in rats by feeding them an 8% casein diet supplemented with 0.3% L-threonine. Rats fed on an amino acid imbalanced diet showed decreased levels of glutamic acid and gamma amino butyric acid (GABA) in brain. But the glutamine levels in the brain was found to be elevated under the same condition. It is suggested that the decreased glutamic acid level in brain might arise from an altered plasma amino acid pattern induced by ingestion of an amino acid imbalanced diet, and the decreased GABA and increased glutamine contents in the brain under the same condition could possibly be attributed to diminished activities of glutamic acid decarboxylase and glutaminase, respectively.