Reversal of insulinopenia, hyperglycemia, glycosuria, and polyuria associated with severe alloxan diabetes in the rat was accomplished by syngeneic transplantation of whole late-gestation fetal rat pancreata. Intravenous glucose tolerance test (GTT) revealed an improved yet still abnormal glucose and insulin response in reversed recipients reconstituted with as few as two pancreata from fetal donors. Eight fetal donors were sufficient to return glucose and insulin response following GTT to normal. Seventy to eighty percent fewer donors were required when the pancreata were transplanted in their entirely as opposed to transplantation of pancreata subjected to prior enzymatic and mechanical dissociation. The facility and simplicity of the whole fetal pancreas implantation technique makes it an appealing model for further study of islet growth and differentiation at the transplant site and of its effect on the metabolic state of the recipient.