To establish a safe and effective usage of oral tacrolimus (FK506) in small bowel transplantation (SBTx) recipients, trough levels and area under the curve (AUC) values of FK506 were assessed using swine models of SBTx and short bowel. Thirty-eight Landrace male piglets were divided into four groups as follows: Group 1, controls (n=13); Group 2, a one-third small bowel model (n=5); Group 3, a short bowel model (n=10); and Group 4, a one-third small bowel allograft model (n=10; five donors and five recipients). Piglets of Groups 1 and 3 were further divided into two sub-groups, according to the route of drug administration: Groups 1a (n=10) and 3a (n=7) received FK506 orally, and Groups 1b (n=3) and 3b (n=3) received FK506 intravenously. Oral or intravenous FK506 was started on post-operative day 3 and continued until day 7 in each group. On day 7, trough levels and AUC values were measured. In Groups 1a, 2, 3a and 4, trough levels of FK506 were 2.1+/-1.2 (p<0.01 vs. Group 2, 3a or 4), 11.2+/-2.1, 23.3+/-4.8 (p<0.05 vs. Group 2 or 4) and 14.6+/-3.0 ng/mL, and AUC values were 101+/-90 (p<0.01 vs. Group 3a or 4), 319&+/-155, 808+/-200, and 531+/-113 ng.h/mL, respectively. Both trough levels and AUC values were lowest in Group 1a and highest in Group 3a. Between Groups 1b and 3b, there was no significant difference in the blood levels of intravenous FK506. The shorter the functioning residual small intestine was, the higher the trough level of oral FK506 was, while the presence or absence of small intestine did not affect blood levels of intravenous FK506. These results suggest that oral FK506 is metabolized in the functioning small intestine during its absorption. Therefore, events which cause intestinal malfunction, such as graft rejection in SBTx, inflammation and loss of small intestine, may adversely raise the trough level of oral FK506.