In the control theories, aging is under genetic and environmental control. Endocrine function plays an important role in this control system by mediating between the environmental influence and the presumptive "aging gene". Therefore, the intrinsic aging of the hypothalamus, such as the changes in sensitivity to feedback suppression or stimulation, may lead to homostatic failure and then age-related pathology. As the subject of study we have selected the senile changes in the hypothalamic-pituitary-ovarian axis in the rat of the Wistar strain. The cessation of estrous cycle and the onset of persistent estrus or repetitive pseudopregnancy usually take place as early as at the end of the first half of life in rats. In this paper the results of the following experiments are briefly dealt with: (i) reciprocal transplantation of ovaries between young and old rats (the term "old" designates here "incapable of reproduction"), (ii) comparison of LH and FSH binding abilities in the ovarian preparations, (iii) comparison of serum and pituitary concentrations of LH, FSH and prolactin and the modifications after ovariectomy or by the administration of pharmacological drugs, and (iv) the difference between young and old rats in intensity of dopamine fluorescence in the hypothalamus. The results of these experiments seem to point to the hypothalamic-pituitary part rather than more peripheral organs (ovaries) as being primarily responsible for the outcome of the senile changes in the female rat.