Over the past 35 y, the U.S. Department of Energy and its predecessor agencies have sponsored studies using beagle dogs to determine the effects of radiation, administered in various forms, by various routes, at various levels of intensity, and in various patterns of exposure, with observations continuing for the life of the animal. These studies have been conducted at several laboratories; some are completed, others are still in progress. Taken as a whole, these studies constitute an experiment of potentially enormous significance to the more complete understanding of the biological effects of radiation.