The aim of this study was to investigate what the consequences of induced drug metabolism, caused by St. John's wort (SJW, Hypericum perforatum) treatment, would have on the plasma, biliary and urinary pharmacokinetics of finasteride and its two previously identified phase I metabolites (hydroxy-finasteride and carboxy-finasteride). Twelve healthy men were administered 5 mg finasteride directly to the intestine via a catheter with a multi-channel tubing system, Loc-I-Gut, before and after 14 days SJW (300 mg b.i.d, hyperforin 4%) treatment. Bile samples were withdrawn via the Loc-I-Gut device from the proximal jejunum. LC–ESI-MS/MS was used to analyze finasteride and its metabolites in plasma, bile and urine. HPLC-UV was used to analyze hyperforin in plasma. The herbal treatment significantly reduced the peak plasma concentration ( C max), the area under the plasma concentration–time curve (AUC 0–24h) and the elimination half-life ( t 1/2) of finasteride. The geometric mean ratios (90% CI) were 0.42 (0.36–0.49), 0.66 (0.56–0.79) and 0.54 (0.48–0.61), respectively. Finasteride was excreted unchanged to a minor extent into bile and urine. Hydroxy-finasteride was not detected in plasma, bile or urine. Carboxy-finasteride was quantified in all three compartments and its plasma pharmacokinetics was significantly affected by SJW treatment. Hyperforin concentration in plasma was 21 ± 7 ng/ml approximately 12 h after the last dose of the 14 days SJW treatment. In conclusion, SJW treatment for 2 weeks induced the metabolism of finasteride and caused a reduced plasma exposure of the drug. New knowledge was gained about the biliary and urinary excretion or the drug and its metabolites.