The brain levels of serotonin (5-HT), norepinephrine (NE) and dopamine (DM) were studied in amino acid imbalance, induced in rats by feeding them an 8% casein diet supplemented with 0.3% L-threonine. In addition, tryptophan hydroxylase (TPH) and monoamine oxidase (MAO) activities in the brain were measured. The studies revealed that induction of amino acid imbalance decreased the brain levels of 5-HT and NE, and increased the DM level significantly. These changes in the amine levels were found to be accompanied by increases in the activities of TPH and MAO of brain. It has been suggested that the enhanced activity of brain TPH in threonine imbalance was probably brought about by a conformational change of the enzyme protein and might be an adaptive response to the low level of 5-HT in the brain due to increased MAO activity. The increased brain DM level in threonine imbalance could be ascribed to enhanced entry of tyrosine in the catecholamine anabolic pathway, while the diminished brain NE level under the same condition might result from its increased degradation or diminished conversion of DM to NE due to supposedly inhibition of DM-beta-hydroxylase.