Transplantation of pancreaticoduodenal allografts was done in 30 totally pancreatectomized dogs. Ten hosts served as a control group and the remaining 20 were treated with azathioprine for immuno-suppression. Excepting two hosts (one in each group) which died of anastomotic complications, the control hosts lived for 3–20 days (averaging 10 days) and the azathioprine-treated hosts for 7–63 days (averaging 22 days). In both groups, hyperglycemia occurred rarely and only after a preponderance of islets were destroyed by rejection; it was therefore considered a terminal event. Serum amylase increased sharply at the start of rejection in control hosts and decreased rapidly to the preoperative level once the initial phase of rejection had passed. In azathioprine-treated hosts, small retarded rises in serum amylase were observed. Histologically the rejection patterns in the allografts were either acute or chronic. A chronic rejection pattern was found only in such allografts as stayed for more than 3 weeks in azathioprine-treated hosts. In the transplanted pancreas, there was no difference between the resistance of acinar cells and that of islet cells against the host's immunological attack. Different parts of the allograft showed different susceptibilities to the host reaction. The parenchyma of the pancreas and of the duodenum, as organs, succumbed readily to the immunological attack. Some lymph nodes were relatively resistant. Nerves and blood vessels maintained their histologic integrity longer than other tissues of the allograft.