From abstract: "In an attempt to gain some insight into the physiological condition of mice surviving massive doses of radiation, a study of their vitality was undertaken. It was assured that the ability of these animals to do forced work would be a measure of their vitality, Stimulating exposed animals to run in exercise wheels to their fullest capacity, has resulted in a demonstration of a striking vitality less during a post-irradiative period, when no other effects are demonstrable by gross examination. Mice subjected to a medium-lethal dose of fast neutrons show a continuous vitality loss over a 300 day period following exposure. Control animals show no appreciable loss of vitality up to about 8 months of age, after which a gradual loss can be noted, probably due to a normal gereologic process."