For an oral herbal medicine, the components that can enter the systemic circulation may be the really effective components. In the present study, the effects on the human cytochrome P450 activities of ginsenoside Rb (1) and two hydrolysis products of 20( S)-protopanaxatriol ginsenosides in humans, namely ginsenoside Rh (1) and F (1), which may reach the systemic circulation after oral administration of ginseng extract, were evaluated. Our results showed that Rb (1) exhibited no marked effects on the activities of human cytochrome P450, whereas Rh (1) and F (1) exhibited competitive inhibition of the activity of CYP3A4 with K(i) values of 57.7 +/- 9.6 microM and 67.8 +/- 16.2 microM, respectively. F (1) also exhibited a weaker inhibition of the activity of CYP2D6. Rh (1) exhibited a weak stimulation rather than an inhibition of the activity of CYP2E1. The degradation of ginsenosides in the gastrointestinal tract may play an important role in the ginseng-associated drug-drug interactions, but the effects might be not due to Rh (1) and F (1).