Peromyscus leucopus, with an average lifespan of 48 months, showed unchanged levels of serum luteinizing hormone (LH), estradiol, progesterone, and pituitary LH and prolactin, between the ages of 12 and 48 months. Hypothalamic LH-releasing hormone (LHRH), norepinephrine and dopamine also remained unchanged with advancing age. Ovarian and uterine weight decreased with age, although the changes in uterine weight were not statistically significant. These data indicate that the hypothalamic-pituitary-ovarian axis remains intact with increasing age, accounting for the maintenance of fertility in these animals. The lack of significant changes in these parameters is in very marked contrast to those in the aging laboratory mouse and rat, which show derangements in their reproductive systems midway through their lifespans.