[en] The Collaborative Radiological Health Laboratory (CRHL) was established in 1962 by the U.S. Public Health Service and Colorado State University for the purpose of determining in a carefully controlled experiment the lifetime hazards associated with prenatal and early postnatal exposure to ionizing radiation. The CRHL study is designed to provide information that will facilitate the evaluation of risks to humans from medical exposure during early development. The study is a long-term (lifespan) study of a moderately large and long-lived mammal exposed at one of several times during development to a relatively small and discrete dose of external radiation. Ages at irradiation selected for comparison reflect the primary concern with medical exposures during the developmental period. The basic experiment under this contract contains 1680 beagles that will be maintained and evaluated for most of their natural lives. Commitment of animals began in December 1967 and was completed in October 1972. This annual report summarizes the current status of the study for the reporting period of January 1 through December 31, 1981