A GC-MS method for the investigation on the metabolism of n-butyl phthalide (NBP) is described. After oral administration of NBP to rats, urine was collected, hydrolyzed with beta-glucuronidase, extracted and concentrated for TMS derivatization, and then analyzed by GC-MS. HBP and its four oxidative metabolites were determined in 0-24 h, 24-48 h rat urine. The mass spectra of the metabolites and their derivatives were presented and the in vivo metabolic pathway was discussed.