This research aims to identify the main active compounds of Rhei undulati Rhizoma (roots of Rheum undulatum LINNE) and determine the types of anthraquinones absorbed into the body and their pharmacokinetic parameters. The boiling-water extract of the herb was administered to 12 healthy volunteers (9 men/3 women) at a dosage of 100 mg/kg the anthraquinone levels in plasma were determined with TLC, HPLC, and LC-MS. Rhein was the only anthraquinone compound absorbed by the body as determined for plasma analysis of the volunteers. The elimination rate constant of rhein in Rhei undulati Rhizoma was 0.23+/-0.02/h and the half life was 3.38+/-0.35 h. This experiment confirmed that rhein is the most important active compound absorbed by the body among anthraquinones contained in Rhei undulati Rhizoma, indicating that rhein is a promising marker substance to evaluate Rhei Rhizoma and Rhei undulati Rhizoma.