The objective of this study was to investigate the effect of tacrolimus (Tac) and xipamide (X) on mycophenolic acid (MPA) glucuronidation in precision-cut rat liver slices. To assess a possible effect of these two drugs, the influence of the anti-inflammatory drug niflumic acid (NA)--a well-known inhibitor for MPA glucuronidation in human liver microsomes--was used as a standard. MPA and its main metabolite mycophenolic acid glucuronide (MPAG) were determined by means of high-performance liquid chromatography. MPA glucuronidation rate showed a significant linear correlation (p = 0.012) with MPA concentrations from 15.61 up to 124.88 microM in the medium. That means, the enzyme(s) responsible for the glucuronidation of MPA worked far below Km-value. With all MPA concentrations tested, neither the addition of Tac (31.30 nM) nor of X (28.25 nM) influenced the glucuronidation of MPA. In comparison, NA at a concentration of 70.92 nM showed a marked inhibitory effect (by 72%). The present pilot-study indicates that precision-cut rat liver slices are a suitable in vitro model to characterize the glucuronidation of MPA to its primary metabolite MPAG and interferences with other substances.