Mice, guinea pigs and rabbits were given (during their life span or for certain limited periods) whole body irradiation at the dose rates of 8.8 r, 4.4 r, 2.2 r, 1.1 r and 0.11 r given in 8 hrs., or 24 hrs./day. Results are as follows: There was a decrease in the life span of mice for the higher dose rates (rabbit and guinea pig data not yet complete) and in mice also for the dose rate of 0.11 r. Significant increases in weight were noted in mice and guinea pigs, the mechanism of this is obscure and apparently not due to castration effects. The hematopoietic system of rabbits and mice is very radioresistant (maximum accumulated doses for mice 5900 r and for rabbits 12000 r (air)) while the effects on the hematopoietic system of guinea pigs is pronounced. The majority of the animals exposed to 8.8 r, 4.4 r, and 2.2 r died of pancytopenia (accumulated doses ranging from 700 to 4400 r). The following tumors were observed in the experimental mice as well as in most control animals: lym-phoma, lung tumors, mammary carcinomas, ovarian tumors and hepatomas. Irradiation mainly shifts the time at which the tumors appear to an earlier age indicating a dependence upon dose rate. The exception are the ovarian tumors the incidence of which is independent of dose rate and depends on a minimum dose. This minimum dose is accumulative even in the animals exposed to 0.11 r. While the effects of irradiation on mouse ovaries are irreversible they are reversible for mouse testes. No conclusive evidence for the production of genetic changes nor of translocations were obtained. This may be due to the comparatively low dose received by the mature sex cells during chronic irradiation. No greying or loss of hair, no atrophy of the skin and no opacity of the eyes were observed. Conclusions drawn from these data as to the problem of human protection against chronic irradiation are as follows: The blood picture is of little value in determining radiation damage. Human ovaries although somewhat less sensitive than mouse ovaries develop tumors similar in type to mouse tumors induced by irradiation. It is therefore suggested to decrease the permissible daily dose for women to 0.02 r. So far as genetic changes are concerned, the data indicate that the present permissible dose of 0.1 r/day gives a sufficiently wide margin of safety that a detectable increase in rate of gene mutations and translocations will not occur.