The total number of oocytes (healthy and atretic) in XO and XX mice, at ages ranging from 12-200 days after birth, was determined using a quantitative histological technique. For each oocyte the stage of the enveloping follicle was recorded. Throughout the period studied XO mice had approximately half the number of oocytes found in XX mice. No increase in the incidence of atresia was detected in XO ovaries. In XO mice at 12 days of age the establishment of the population of growing follicles was clarly retarded.