[en] The lifetime hazards associated with exposure to ionizing radiation during development are studied in 1680 beagle dogs given whole-body exposures to ⁶⁰Co gamma radiation. Eight groups of 120 dogs each received 20-R or 100-R exposures at 8, 28, or 55 days postcoitus (dpc) or at 2 days postpartum (dpp). In addition, exposures of 100 R were given to 120 dogs at 70 dpp and to 240 dogs at 365 dpp. An additional 360 dogs were sham exposed. Smaller groups of dogs were used to identify organs and tissues of particular sensitivity to radiation injury during development and to evaluate mechanisms of radiation injury. The research is concerned primarily with evaluating the role of age at exposure as a factor influencing response to radiation injury. As of December 31, 1982, of the 1680 dogs, 1058 were dead. Survivors ranged from 9.9 to 15 years of age. Through 10 years of age, no differences in survival were evident in any exposure groups. A variety of clinical, pathophysiologic, and pathologic responses have been studied. Irradiation during development has been found to be associated with abnormalities of skeletal, dental, and central nervous system development. Irradiation during ocular development has induced dysplastic and atrophic retinal lesions. Perinatal irradiation of the kidney has resulted in dysplasia, and, in animals receiving higher doses, significant chronic renal disease. The thymus gland, particularly thymic epithelium, has been found to be highly radiosensitive during fetal development