Recent data point to a significant role for thymosins, lymphokines, and other soluble mediators in the senescence of the immune response that occurs with aging. In recent years, considerable progress has been made in the isolation and physiochemical characterization of several of these soluble mediators. We are now beginning to define the mechanisms by which these molecules regulate and mediate immune responses. In this paper we review the properties of the best characterized thymic hormones and lymphokines and focus on the role of the endocrine thymus in modulating immune responses. Of particular interest is the recent observation that thymosin fraction 5 can enhance production of interleukin-2 (IL-2) and colony-stimulating factor (CSF), and that IL-2 production, but not CSF production, is selectively diminished in aging mice. Several of the products of the immune system also can act as neuroactive immunotransmitters and modulate a number of neuroendocrine responses. Current studies point to an important role for these molecules in modulating neuroendocrine function, suggesting a broader role for the endocrine thymus in the aging process.