SUMMARY 1. The ovaries of rabbits and mice were irradiated by means of a superficial X-irradiation apparatus. For rabbits the total dosage varied between 800 and 2000r, and for mice between 300 and 1500r. All the rabbits, but only about one in four of the mice, survived the treatment. 2. One ovary was removed from the ten irradiated rabbits and from twenty-seven of the surviving mice. Control animals were also unilaterally ovariectomized. The rabbits were then left from 34 to 139 days, and the mice from 61 to 115 days. All the irradiated rabbits and fourteen of the irradiated mice survived for the duration of the experiment. 3. Seven of the rabbits and ten of the mice proved to have ovaries which had been sterilized by means of the X-irradiation. Compensatory ovarian hypertrophy did not occur in any of these animals. It did occur in the controls and in the animals in which sterilization was incomplete.