We recently evaluated the acetaminophen absorption test as a marker of graft rejection for small bowel transplantation(SBTX). Randomly bred male Wistar rats were used as recipients and donors. Rats (n=45) received heterotopic small intestinal transplants and were divided into three groups (n=15 for each group). In group A, a 10-cm segment of jejunum of was exteriorized as a Thiry-Vella loop. In group B, immunosuppression was not given after SBTX. In group C, rats were treated with FK506 after SBTX (0.3 mg/kg body weight, 0≈6 postoperative days). Serum acetaminophen concentrations were measured 15 min after instillation of 0.15 g/kg acetaminophen into the intestinal loop on postoperative days 1, 3, and 7 (n=5 for each group). Blood flow and histology of the graft were also evaluated. In the SBTX group only, the grafts showed the histological change after acute rejection. On day 3, plasma acetaminophen concentrations in this group showed a significant decrease, which correlated with the mild histological changes of graft rejection. Graft blood flow of the SBTX group decreased significantly on day 7, following the severe graft destruction of advanced rejection. No remarkable changes were observed in the other two groups. The acetaminophen absorption test appears to be useful for the early detection of SBTX graft rejection.