An automated pre-column derivation procedure has been developed for the determination of gamma-aminobutyric acid (GABA) and 17 free amino acids by means of reversed-phase high performance liquid chromatography. GABA and the 17 free amino acids in needles of Pinus massoniana seedlings were determined with a 0-phthaldialdelhyde-9-fluore-nylmethyl chloroformate pre-column derivation in 19 min. The amino acid derivatives were separated on an Agilent Hypersil AA-ODS-C18 column by a gradient elution method and identified with a DAD detector. The recovery of amino acids by this method was more than 90.1% with high precision and reproducibility (its relative standard deviation ranged from 0.21% to 281%). The total amount of amino acids in the needles of P. massoniana seedling decreased decidedly from 418.3 to 310.4 μg?g-1 FW after insect damage.