The plasma concentrations of free (unconjugated) and conjugated silybin after intake of a single oral dose of a lipophilic silybin-phospatidylcholine complex (silipide, 80 mg expressed as silybin equivalents) were evaluated in 12 healthy volunteers by using a sensitive and specific HPLC method. Free silybin concentrations reached a peak of 141 +/- 31 ng/ml (mean +/- SEM) at 2.4 hours after dosing and declined thereafter with a half-life of about 2 hours. Peak concentrations of conjugated silybin were greater (255 +/- 35 ng/ml) and occurred at a later time (about 3.8 hours). The elimination of conjugated drug tended to be slower than that of free drug. AUC values for conjugated sylibin were about three-fold greater than those of free drug. It is concluded that after oral intake of silipide, silybin undergoes extensive conversion to conjugated derivative(s) which are retained in the circulation at relatively large concentrations.