Median survival time (MST) and major causes of death were determined for 360 control and x-irradiated female beagles. At 10 to 12 months of age the beagles were given single or fractionated x-ray (250-kVp) exposures totaling either 100 R or 300 R. Thereafter they were maintained in outdoor pens for lifetime studies. Most deaths occurring during the first 3 years postirradiation appeared to be due to synergism between the effects of environmental stresses and x-irradiation. In the final analyses, all irradiated beagles exhibited life-span shortening relative to controls: 9.5% and 20.7% for the 100-R and 300-R groups, respectively. On a linear scale this represented a loss of 6.7% per 100 R. Differences between subgroups receiving fractionated exposures were apparent only at 300 R, where life-span shortening was reduced when the elapsed time between first and last exposure was increased from 7 to 84 days. Mortality rates could be fit to the Gompertz equation only during the last 6 years of life. The Gompertzian slopes were similar for all groups, except that irradiated groups attained higher mortality rates earlier in life than controls. Neoplasms and chronic diseases were the major causes of death, and these entities were similar in controls and irradiated beagles. Results from this study agree quite well with those obtained from rodents.

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