The purpose of the study was to investigate intestinal mucosal perfusion in mouse small-bowel transplantation (SBT), using laser-Doppler flowmetry. Heterotopic SBT was performed in syngeneic and allogeneic combinations. Mucosal perfusion was measured both in the native and in the grafted intestine at time of surgery and at 1, 3, 6, and 8 days postoperatively. Histology specimens were obtained at the same time and graded for rejection. No rejection was seen in the syngeneic group at any of the time points studied. The allografts displayed significant decreased mucosal perfusion on postoperative days 3, 6, and 8. Rejection was histologically evident on postoperative days 6 and 8. Laser-Doppler perfusion in the rejecting intestinal allograft was decreased before onset of histological features of rejection. Mucosal blood flow measured by laser-Doppler could be used as an early indicator of acute rejection in SBT.