It has been reported that dietary restriction and chemical blockade of iodine causes histopathologic changes in peripubertal female rat breasts. This study extended the age range to include midreproductive life and perimenopausal rats; there is a wider spectrum of structural alterations that are associated with the older breast, with sodium perchlorate as the blocking agent. In 16-week-old rats, breasts showed general increased parenchymal activity and growth, regressing after removal of the block. In 42-week-old rats, breasts showed noticeable calcospherite deposition, intralobular fibrosis, and cystic changes resembling human fibrocystic disease. In 52-week-old rats, breasts exhibited atypical lobules cytologically, papillomatosis, sclerosing adenosis, calcifications, and a lobular transformation of a histologically dysplastic type. It is the older rat that experiments will more closely parallel the human condition.