The effects of Senkyu (Cnidii Rhizoma and Ligustici chuanxiong Rhizoma) on the drug skin penetration were studied to clarify its effectivity as the baths. Ether and methanol extracts, and some essential oils of Senkyu (i.e. ligustilide, neocnidilide and butylidenephthalide) enhanced remarkably the skin penetration of benzoic acid. Furthermore, an appreciable correlation between the enhancing ratio and the skin/donor partition coefficient of benzoic acid was observed. These facts suggest that the constituents of Senkyu influence the skin penetration by enhancing the partition coefficient.