1. Four-day-old male and female Long-Evans x Sprague-Dawley F1 hybrid rats were exposed to a single total body x-ray dose of 125, 25, 5, and 0 r and then were observed during their entire lifespan for weight changes, signs of morbidity and age at death. 2. There was a 10% reduction of life span in the 125 r exposed female rats. None of the male groups showed significant shortening of longevity. 3. Mean body weights were consistently lower in both male and female groups exposed to 125 r total body irradiation. 4. Female rats exposed to 25 and 125 r attained their maximum weights at a younger age than their non-irradiated controls. 5. External tumors appeared earlier in female rats. Tumor incidence was not increased by irradiation in either sex. 6. Male parentage influenced the length of life span of progeny to a greater degree than the irradiation exposure at the employed dose-level. The influence of female parentage upon longevity cannot be evaluated in this study. 7. Low dose radiation life shortening can only be demonstrated in long-lived strains. Hereditary and radiation life shortening effects are not cumulative under conditions of this study.