The pharmacological effects of Oren-gedoku-to (OGT), a Japanese-Chinese traditional herbal medicinal mixture on lipid biosynthesis were investigated in cultured human hepatocyte HepG2 cells. The addition of OGT (0.5 and 4.2 mg/ml), which had no effect on cell proliferation and cellular protein content, caused a marked decrease in the cellular cholesterol content, particularly cholesteryl ester content following 24 h incubation. The incorporation of 14C-oleate into cellular cholesteryl ester fraction was also reduced remarkably during incubation for 6 and 24 h. The effects of OGT, its components and its main active chemicals on acyl-coenzyme A:cholesterol acyltransferase (ACAT) activity were studied in vitro to explore the mechanism by which OGT inhibits cholesteryl ester formation. The data confirmed that OGT, in a dose-dependent manner, and its components (Scutellaria baicalensis, Coptis japonica, Gardenia jasminoides and Phellodendron amurense) remarkably inhibit ACAT activity. Among the main active chemicals of OGT, baicalein, a kind of flavonoid, decreased ACAT activity in a dose-dependent fashion from the level of 10(-6)M. These results strongly suggest that OGT reduces the cholesteryl ester formation in human hepatocytes by inhibiting ACAT, and that baicalein may, in part, be responsible for ACAT inhibition.