The effect on the ovary of a single dose of 20 r X-ray given at different ages early in life varies according to the age of the animal at the time of radiation. This has been investigated by determining the number of oocytes surviving 24 hr after radiation, and at varying intervals after irradiation but at a constant age of the animal, i.e. 49 days. The sensitivity of the ovary depends on the response of the small oocytes as well as on the response of the growing and large oocytes. The sensitivity of these two groups differs. Further, a variation within the two groups is noted, which is age dependent. Fifty per cent of small oocytes survive 24 hr after radiation on the day of birth whereas radiation after this age leaves only between 1 and 9% of these cells intact. The number of oocytes in the ovary at the time the animal enters maturity is 85% of the normal number after irradiation at birth, but only 1% after irradiation at the age of 3 weeks. The changing radiation sensitivity is discussed in relation to certain morphological changes in the developing ovary.