Random bred Schofield albino mice were unilaterally ovariectomized or sham-operated at 6 weeks of age and were killed 4, 8, 16 or 32 weeks later. Ovarian weights were recorded before quantitative histological analysis of the total number of oocytes per ovary, the number of follicles per size class, and the proportion of oocytes undergoing atresia. Ovarian weights were consistently higher in the unilaterally ovariectomized mice than in coeval controls and by the 8th week after surgery the weight of the remaining ovary approached the combined weights of the two ovaries in controls. On a per ovary basis, follicular 'pool' size was reduced and the number of growing follicles increased after unilateral ovariectomy, these changes occurring within the first 4 weeks after surgery. Thereafter, oocyte depletion occurred at the control rates. There was no change in the rate of atresia after surgery.