Ischemia-reperfusion injury is an unavoidable problem for organ transplantation including small bowel transplantation, and causes a large intra-individual variation of tacrolimus (FK506) pharmacokinetics. Little information is available about the regulation of the intestinal P-glycoprotein expression during tissue regeneration. In the present study, we have examined the molecular and functional variations of ileum P-glycoprotein using rats after ischemia-reperfusion treatment. Morphological study revealed a rapid regeneration of the intestinal wall during 24 h after reperfusion. A reverse transcription-coupled competitive PCR and Western blot analysis revealed that the intestinal expression of P-glycoprotein recovered with time after reperfusion. At 24 h after reperfusion, the ileum P-glycoprotein level was transiently increased to two-fold, and the absorption rate of dihydro-[(3)H]FK506 from in situ ileum loop into portal vein was markedly low in comparison with the control. P-glycoprotein was detected in the crypt area as well as in villous cells at 6 h after reperfusion, and then localized to the apical surface at 24 h consistent with the cell proliferation and differentiation. However, the P-glycoprotein level returned to normal at 48 h. The intra-individual variation in the absorptive rate of tacrolimus was suggested to be regulated by the morphological status of the intestinal epithelium and enterocyte expression level of P-glycoprotein. Therefore, the monitoring of the enterocyte P-glycoprotein level would provide useful information for determining the dosage of tacrolimus immediately after small bowl transplantation.