The immune, neuroendocrine, and central nervous systems are stimulus response systems that are similar in the functions they subserve and tightly integrated in their actions. The reciprocal regulatory effects of these systems provide a basis (but not proof) for the belief that brain-immune interactions are of clinical relevance and not reducible to characteristics of component systems. In this paper, evidence for the integration and mutual regulation of central nervous and immune systems is reviewed. Additionally, behavioral and time-dependent neuroendocrine effects of interleukin-2 are presented in detail.