MBH tissue, which included the hypothalamic arcuate nucleus of fetal or neonatal rats, was transplanted into the third ventricle of aged (21-30-month-old) female rats. The brain and ovaries of each recipient were examined histologically 3 or 4 weeks after transplantation. Four grafted MBH tissues were examined ultrastructurally and immunohistochemically 4 weeks after transplantation. The appearance of the MBH grafts was similar to that of normal neural tissue. The neuropil in the grafts was fully occupied with numerous axons, dendrites, and glial processes. A number of axodendritic shaft and spine synapses were observed in the neuropil. Immunohistochemical analysis with antiserum to TH revealed stained (immunoreactive) neuronal perikarya and processes in the grafts. TH-immunoreactive processes originating from the TH-positive neurons in the grafts could be seen to extend across the graft-host interface. The ovaries of six out of nine females that received MBH grafts exhibited follicles of various sizes and healthy appearing corpora lutea. On the other hand, some follicles and masses of interstitial cells were prominent in the ovaries of the intact animals or controls that had received cortical grafts. In the females that received MBH grafts, the ovarian weight was significantly greater than that in the controls. These results suggest that the neural substrates in fetal MBH tissue can survive and develop well in the aged rat brain and that MBH grafts may play some role in the recovery of declined ovarian function in aged female rats.