There is evidence to indicate that the neuroendocrine and immune systems can interact. Thus, neuroendocrine hormones can modulate a variety of immune functions and there have been attempts to manipulate the neuroendocrine system of aged animals to enhance immune function. We have previously shown that the transplantation of a syngeneic pituitary gland under the kidney capsule of young adult mice elevates serum prolactin and enhances immune responsiveness. In the present study pituitary glands were transplanted under the kidney capsule of 22-month-old mice to determine if this maneuver can enhance a number of immunologic parameters. The results demonstrate that aged animals bearing transplanted pituitary grafts for 10 days did not exhibit any enhancement in their primary antibody response to sheep red blood cells, splenic T or B-cell mitogen responsiveness or restoration of thymic architecture. When these immunologic assessments are performed on animals bearing pituitary grafts for 28 days, the IgM and IgG primary antibody responses and splenic T-cell responsiveness are enhanced but repopulation of the thymus still does not occur. Importantly, this enhancement does not restore immunocompetence to levels observed in young mice.