Ge-gen (Radix Puerariae) is used in traditional oriental medicine for various medicinal purposes. The drug is the root of a wild leguminous creeper, Pueraria lobata (Willd) Ohwi. It possesses a high content of avonoid derivatives, the most abundant of which is puerarin. Our goal was to find the effect of puerarin on cytochrome P450 enzymes in vivo. The study was conducted in 18 male volunteers of different genotypes (CYP2D6 *1/*1, *1/*10, *10/*10). Plasma was obtained at 6 h after oral administration and urine was collected from 0 to 8 h after probe drug administration. The logarithm value of metabolic rate decrease from -0.0055 +/- 0.1887 to -0.1754 +/- 0.2411 implied puerarin inhibited activity of CYP2D6. There was no significant relationship between the inhibition with the CYP2D6 genotypes. The paraxanthin/caffeine ratio in the plasma sample at 6th hour was increased by 30 +/- 47% (p = 0.003), implied puerarin induced the activity of CYP1A2. While puerarin used together with the substrates of both enzymes, drug interaction worth the attention and at sometimes precautions are needed.