Liquid chromatography-atmospheric pressure chemical ionization-mass spectrometry (LC-APCI-MS) method has been developed for the measurement of the concentrations of 20(S)-ginsenoside Rh1 and its aglycone 20(S)-protopanaxatriol in rat plasma with panaxatriol as internal standard. The method involved single liquid-liquid extraction of both 20(S)-ginsenoside Rh1 and 20(S)-protopanaxatriol from plasma samples with n-butanol. The limit of quantification (LOQ) was 5 ng mL(-1) for both compounds. The method was validated within the linear range 5-2000 ng.mL(-1) for both compounds. The correlation coefficient for the calibration regression line was 0.999 or better. Intra-day and inter-day accuracy were better than 15%. The method has been successfully used for the pharmacokinetic studies in rats. After intravenous administrations, the mean retention times of 20(S)-ginsenoside Rh1 and 20(S)-protopanaxatriol were 17.1 +/- 2.0 min and 3.46 +/- 0.33 h, respectively.