Objective : Herb-drug interactions have become an important issue because of the consumption of herbal remedies has increased in the world. Yangguksanhaw-tang (Liang ge san huo-tang) and Taeumjowi-tang (Tai yin tiao wei-tang) are typical herbal formulas on Sasang constitution medicine (four-constitution medicine). This study was aimed at evaluating the effects of Yangguksanhaw-tang and Taeumjowi-tang on drug metabolizing enzymes, cytochrome P450 (CYP450) isozymes. Methods : Vivid$^{(R)}$ CYP450 Screening Kits were used to measure of CYP3A4, CYP2C19, CYP2D6 and CYP2E1 activities. This method is based on the use of fluorescent CYP450 substrates that are efficiently metabolized by specific CYP450 isozymes to yield a product with altered fluorescent properties. The percent inhibitions of CYP450s by herbal formulas were calculated. Results : Yangguksanhaw-tang inhibited CYP2C19 and CYP2E1 activities higher than that other CYP450 isozymes. The $IC_{50}$ values of CYP2C19 and CYP2E1 were 159.83 ${mu}g/mL$ and 261.40 ${mu}g/mL$, respectively. The CYP2E1 activity was inhibited ($IC_{50}=215.17{mu}g/mL$) higher than that other CYP450 isozymes by Taeumjowi-tang. Conclusions : These results suggest that Yangguksanhaw-tang may inhibit the metabolism of co-administered drugs whose primary route of metabolism is via CYP2C19 or CYP2E1. Taeumjowi-tang could inhibit the metabolism of co-administered drugs, which are substrates for CYP2E1. Therefore, co-administration of the herbal formulas and other conventional drugs should be undertaken with care.