To evaluate the lifetime hazards of exposure to ionizing radiation, 1,680 beagles received whole-body exposures to 60Co gamma rays or sham exposures during development. Eight groups of 120 dogs each received mean doses of 16-18 or 81-88 cGy at 8, 28 or 55 days of gestation, or at 2 days after birth. One group of 120 dogs received a mean of 83 cGy at 70 days of age and one group of 240 dogs received a mean of 81 cGy at 365 days of age. Sham irradiations were given to 360 controls. Sexes were equally represented. In 1,343 dogs allowed to live out their life span, heritable lymphocytic thyroiditis with hypothyroidism was a major contributor to mortality. Irradiated dogs had a decreased risk for hypothyroidism, a finding that was surprising and not easily explained. Of the 1,343 life-span dogs, those exposed as neonates at 2 days of age or as juveniles at 70 days of age had evidence for an increased risk for thyroid follicular cell neoplasia. Hypothyroid dogs had a significantly increased risk for thyroid neoplasia, including greater risk for carcinomas, but no evidence of a greater sensitivity to radiation-induced tumors. In dogs with normal thyroid function irradiated at 2 or 70 days of age there was increased risk for benign and malignant follicular cell neoplasms, including multiple neoplasms. No difference between sexes was noted. These findings related to age sensitivity in the dog were consistent with the high risk for radiogenic thyroid neoplasia in humans after exposure during early childhood.