d-Limonene shows carminative and cholagogue effects and is used in treatment of gallstone, cholecystitis and angiocholitis. A simple method was developed to determine the concentration of d-limonene in human plasma. d-Limonene and internal standard (naphthalane, C(10)H(18)) were extracted with n-hexane and then injected to GC-MS. Calibration curves were linear (r=0.9990, n=6) in the range of 2-500 ng/ml for d-limonene in human plasma. Limit of detection and quantification were 0.5 and 2 ng/ml, respectively. This rapid and specific method was applied to the clinical pharmacokinetic investigation of d-limonene.