[en] This document describes studies on chronic radiation injury in experimental animals and the extrapolation of derived injury parameters to man. Most of the large studies have used mice given single, weekly, or continuous exposure to cobalt-60 gamma rays, or, more recently, single or weekly exposure to fission neutrons from the JANUS reactor. Primary measures of injury have been life shortening and the associated major pathological changes, particularly neoplastic diseases. Recent and ongoing studies compare the effects of extremely low neutron exposures with gamma irradiations delivered as a single dose or in 60 equal weekly increments. Total neutron doses range from 1 to 40 rads; gamma-ray doses range from 22.5 to 600 rads. Selected genetic studies are performed concurrently to provide a nearly complete matrix of somatic and genetic effects of these low exposures. Studies with the beagle have complemented those with mice and have shown a strong parallelism in the responses of the two species. Present exposures are at 0.3, 0.75, and 1.88 rads per day of continuous gamma irradiation to test a model for the prediction of life shortening in man which has evolved from Argonne's long-term studies. The dog offers the opportunity for longitudinal clinical evaluations that are not possible in the mouse, to develop a broader view of the neoplastic disease spectrum, and to study the mechanisms of radiation induction of leukemia. Diverse statistical approaches have been used to measure excess risk, dose-response functions, and rates of injury and repair. Actuarial statistical methods have been favored since they permit a more direct means of extrapolation to man. 50 refs., 4 figs