## Abstract

In a quantitative study of the reaction of the rat ovary to X-irradiation, a curvilinear relationship has been established between the number of oocytes that survives exposure to X-rays, and (i) the dose of irradiation, and (ii) the time after exposure. The speed with which an individual oocyte disappears seems to be largely independent of the dose of X-rays. The population of oocytes decreases very rapidly during the first 18 h after X-irradiation, the rate of decline after 24 h being almost negligible. Primordial oocytes are more radio-sensitive than those in later stages of development. The lowest dose of X-rays which destroys most of the primordial oocytes within 18 h is about 300 r. With the possible exception of Graafian follicles, follicles in later stages of development require much higher doses of irradiation before degenerating. The tolerance of 'medium-sized' follicles would thus appear to be responsible for the fact that the dose which is necessary to sterilize adult rats is extremely high (4400 r). The results also show that the proportion of oocytes destroyed by a given dose of X-rays depends partly on the age of the animal at the time of exposure. The younger the animal, the higher the proportion of oocytes which are eliminated. Multiple regressions have been calculated according to the formula $y=a+b_{1}x_{1}+b_{2}x_{2}+b_{3}x_{3}$, where $y=\log _{10}$ count of oocytes $\times 10^{2}$, $x_{1}=\log _{10}$ age in days $\times 10^{2}$, $x_{2}=\log _{10}$ dose of X-rays in $\text{r}\times 10^{2}$ $\text{and}$ $x_{3}=\log _{10}$ post-operative period in $\text{h}\times 10^{2}$. It is thus possible, by means of regression coefficients $a,b_{1},b_{2}$ $\text{and}$ $b_{3}$, to obtain an estimate of the numbers of oocytes in different stages of development which are likely to survive after the ovaries have been exposed to different doses of X-rays.