This study was designed to investigate whether in combined stomach, small bowel, and pancreas transplantation allograft rejection occurs in the individual organs concomitantly and with the same intensity. Heterotopic en-bloc transplantation of the stomach, small bowel, and pancreas was performed in a Lewis-to-Brown Norway rat combination. Group 1 animals received no immunosuppressive therapy while animals in group 2 were treated with cyclosporin (10 mg/kg body weight, orally) daily. Grafts were histologically evaluated on the 5th (subgroups 1a and 2a) and 10th (subgroups 1b and 2b) postoperative days. The degree of rejection was defined as moderate, intermediate, or severe according to predefined criteria. The results indicate that the small bowel is more susceptible to rejection than either the stomach or the pancreas. Mucosal biopsies of the stomach are unlikely to provide a reliable guide to rejection in the small bowel.