The objectives of this study were to identify whether there is a decline in Mullerian inhibiting substance (MIS) in the female rat during chronological aging, and to define the physiological basis of aging-related changes in MIS. The results demonstrate that there is an exponential decline in both serum and ovarian levels of MIS with increasing female age, and that the histologic origin for the reduction in serum levels of MIS is a decline in the number of small ovarian follicles expressing MIS.