We recently reported that the blood concentrations of Tacrolimus (FK506) in rats were markedly increased following the intake of a Chinese herbal preparation, Wuzhi Tablet (WZ, Schisandra sphenanthera extract). In order to identify the underlying mechanisms of the increase in FK506 level, we investigated the effects of WZ on the absorption and first-pass intestinal and hepatic metabolism of FK506 in vitro and in vivo. When co-administered with WZ, the AUC0-∞ value after oral FK506 dosing was increased by 2.1 fold, the oral bioavailability (Foral) was increased from 5.4% to 13.2% (p = 0.0002), and the (Fabs × FG) was 111.4% (p < 0.01), much greater than that when FK506 was given alone. However, the FH was only 21.2% greater than that when FK506 was given alone, which indicates that the reduction of intestinal first-pass effect was the major cause of the increased FK506 oral bioavailability when co-administered with WZ. In the Caco-2 cell transport study, the transport ratio of FK506 with WZ extract was significantly lower than that of FK506 alone, which suggested WZ extract inhibited P-gp-mediated efflux of FK506. Furthermore, 100 μM of WZ extract almost completely inhibited FK506 metabolism in rat and human liver microsomes, indicating WZ extract potently inhibited the CYP3A-mediated metabolism of FK506. In conclusion, WZ inhibited P-gp-mediated efflux and CYP3A-mediated metabolism of FK506, and the reduction of intestinal first-pass effect by WZ was the major cause of the increased FK506 oral bioavailability. © 2010 Elsevier B.V. All rights reserved.