The CYP3A4 substrate budesonide was used to investigate gut wall first-pass metabolism in jejunum, ileum and colon of eight healthy men. Three milligram budesonide in solution was installed at each location by intubation, with or without immediate prior local administration of 16mg ketoconazole. Simultaneously, deuterium-labelled budesonide (0.2mg) was administered intravenously. Pharmacokinetics of unlabelled and labelled budesonide in plasma was evaluated using LC-MS/MS. Ketoconazole increased budesonide systemic availability significantly in jejunum (from 11.8 to 21.7%) and ileum (from 15.9 to 31.8%). T(max) and MAT were unaffected. No significant effects were noted after colon administrations, nor were there any effects on the pharmacokinetics of intravenously administered budesonide. The increased bioavailability is most probably explained as inhibition of gut wall metabolism. The data are in accordance with the well-known CYP3A activity in the small intestine, evidently capable of metabolising at least half the absorbed dose of budesonide. In contrast, the results indicate that this enzyme activity is insignificant in the colon.