A transplantable insulinoma was developed in inbred albino rats of the NEDH strain. The original tumor, 1 cm in diameter, was removed from the pancreas of a male parabiont 566 days folowing 1000 rads (10J/kg) of total body x-irradiation. The time required for implanted fragments to grow to 0.5-1.5 cm in diameter decreased from 5-8 months in the first generation to 2-5 months in the seventh generation. Successful transplantation in male animals followed for 4 or more months after transplantation was significantly greater than in female animals followed for a similar period of time (96% versus 69%). Light and electron microscopy revealed that the tumors consisted predominantly of well-granulated beta cells. Ultrastructural studies also showed small numbers of D-cells. Tumor extracts contained an average of 223 units of immunoreactive insulin and 25.9 mug of immunoreactive somato-statin per gram wet weight of tissue. Tumors generally produced increasingly profound hypoglycemia within 2-4 months following transplantation, with plasma glucose levels frequently falling to 40 mg/100 ml or lower prior to death. Removal of tumors from chronically hypoglycemic animals resulted in transient rebound hyperglycemia with plasma glucose levels above 300mg/100 ml within the first 24 hr and a gradual decline to normal levels of 129 mg/100ml in 2-4 days. These observations correlated with findings of marked atropy and degranulation of the beta cells in the pancreata of tumor-bearing animals, and with gradual return of normal light microscopic morphology following tumor removal.