Despite similar immunosuppressive protocols having been applied, the success rate of pancreatic islet transplantation has been much lower than that of wholepancreas transplantation. We compared the oxygen tension in syngeneically transplanted isolated rat islets and islets of syngeneic rat whole‐pancreatic grafts, with native islets, using Clark‐type microelectrodes. An oxygen tension of ∼40 mmHg was recorded in both native islets and in islets of the whole‐pancreas graft. Isolated islets transplanted under the renal capsule had a markedly lower oxygen tension (∼6 mmHg). The exocrine parenchyma of the native and of the transplanted pancreatic gland had an oxygen tension of ∼30 mmHg. The lower oxygen tension in transplanted isolated islets may be one explanation for the more frequent failures of transplanted islets compared with the outcome when the whole pancreatic gland is transplanted.