Hypothalamic and pituitary functions of 21-month-old rats in constant estrus were compared with those of 3-month-old cycling rats on the day of estrus. FSH-RF in the hypothalamus and pituitary FSH were significantly higher in the old than in the young rats. Pituitary LH was significantly lower in the old than in the young rats. Hypothalamic PIF was similar in both groups, but pituitary prolactin concentration was significantly greater in the old rats. The old constant-estrous rats showed well-developed ovarian follicles but no corpora lutea, increased uterine weight, hyperplastic mammary glands, and increased pituitary weight. It is suggested that the constant estrus observed in many old female rats is the result of a fundamental change in hypothalamic control of pituitary FSH, LH, and prolactin secretion.