The effect of nonionic surfactant(polyoxyl 40 hydrogenated castor oil, PHCO), a common solubi-lizer, on the solubility of silymarin in the combined preparation containing ursoseoxycholic acid(UDCA) and silymarin was investigated in vivo using HPLC. The solubility of silybin, a major component of silymarin, was enhanced by increasing the amount of PHCO. The effect of PHCO on bioavailability was also evaluated in rats. The bioavailability was calculated by silybin content in bile juice that was excreted for 24 hr after oral administration. It was found that the bioavailability of silymarin containing PHCO was significantly increased compared to that of control. These results suggest that PHCO may improve the solubility and bioavailabilty of silymarin when it is combined with UDCA and silymarin.