The influence of a chronic environmental stress on the incidence of radiation-induced tumors, lens opacities, and survival times was determined. Female CFN rats were exposed to whole-body irradiation with either 300 rad of fission neutrons, 600 rad of 250 kV X rays, or sham irradiation. Within each irradiated group, rats were assigned to one of four environments: exposure to 25°C until irradiated and immediately returned to (1) a 25°C or (2) a 2°C environment for duration of life; acclimation to a 2°C environment for 40 days, irradiated, then returned to (3) a 2°C or (4) a 25°C environment. Postirradiation exposure to the 2°C compared to the 25°C environment reduced tumor incidence in the X- and neutron-irradiated groups, but reduced incidence of lens opacities only in the neutron-irradiated group. A 2°C environment before and after irradiation produced similar results. Return of the 2°C acclimated rats to a 25°C environment increased incidence of tumors and lens opacities in the X-ray group, but reduced incidence of both in the neutron-irradiated rats. Chronic exposure to 2°C reduced the tumor incidence of nonirradiated rats but did not change the incidence of lens opacities. Median life expectancy was reduced by exposure to the 2°C environment. The data are consistent with the hypothesis that a chronic environmental stress initiated immediately after a carcinogen, in this study X or neutron radiation, may inhibit the malignant transformation. In view of other published data the stress-induced increase in metabolic rate may be an important factor in the inhibition.

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