Parathyroid morphology was studied in 45 irradiated and 37 control rats. The irradiated rats received a single parathyroid dose of about 6.5 gray. The rats were killed and autopsied 14 months later. The radiation resulted in fibrosis and depression in weight of the parathyroids. In irradiated rats eleven parathyroid adenomas and four thyroid tumors were found. In control rats no tumor occurred in either parathyroid or thyroid glands. All irradiated rats, including those with a parathyroid adenoma, were normocalcemic.