Contents of five amino acids (gamma-aminobutyrate (GABA), glutamate, aspartate, glycine, and taurine) were assayed in the rat striatum in which nigrostriatal pathways were unilaterally destroyed by 6-hydroxydopamine. GABA content in the operated side was significantly increased 1 month after the surgery compared with that in the nonoperated side. Contents of four other amino acids assayed did not change significantly. Among possible mechanisms for this elevation we postulated that sprouting of the GABAergic terminals in the striatum would be most probable.