Neonatal rat pancreatic islets were transplanted intraperitoneally into adult streptozotocin-diabetic rats and mice. Isologous pancreatic islet recipients (12 of 12) showed consistent and permanent reconstitution of normoglycemia and normal weight gain as well as readjustment to normal of intake of water, urine volume, and glucose excretion for greater than 10 months when compared to age-matched normal (ten) and diabetic (20) controls. Revascularized isologous islet grafts were found to be adherent to both visceral and parietal peritoneum. Pancreatic islet allografts (ten) and allografts and xenografts did not appear to differ markedly; both were characterized by dense round-cell infiltration within 5 days after transplantation.