Aging in female rodents is associated with a gradual decline in litter size which culminates in infertility. In the C57BL/6 mouse such changes are evident at eight months and gradually increase until, by 15-16 months, most females are sterile. The objective of the proposed research is to determine the histological changes which occur in the ovary and pituitary of mature (3-5 months of age) and aged (12-15 months of age) C57BL/6 mice during days 1 (day of copulatory plug), 4, 9 and 16 of pregnancy using light and electron microscopy, and to associate these changes with the circulating plasma levels of progesterone, estrone, estradiol-17beta, prolactin, luteinizing hormone and follicle-stimulating hormone using radioimmunoassay. The correlation between cytological changes in the ovary and pituitary and circulating levels of hormones during senescence should provide data on the causes of reproductive dysfunction and eventual sterility.