The molecular and biochemical bases for interactions between the immune and central nervous systems are described. Immune cytokines not only activate immune function but also recruit central stress-responsive neurotransmitter systems in the modulation of the immune response and in the activation of behaviors that may be adaptive during injury or inflammation. Peripherally generated cytokines, such as interleukin-1, signal hypothalamic corticotropin-releasing hormone (CRH) neurons to activate pituitary-adrenal counter-regulation of inflammation through the potent antiinflammatory effects of glucocorticoids. Corticotropin-releasing hormone not only activates the pituitary-adrenal axis but also sets in motion a coordinated series of behavioral and physiologic responses, suggesting that the central nervous system may coordinate both behavioral and immunologic adaptation during stressful situations. The pathophysiologic perturbation of this feedback loop, through various mechanisms, results in the development of inflammatory syndromes, such as rheumatoid arthritis, and behavioral syndromes, such as depression. Thus, diseases characterized by both inflammatory and emotional disturbances may derive from common alterations in specific central nervous system pathways (for example, the CRH system). In addition, disruptions of this communication by genetic, infectious, toxic, or pharmacologic means can influence the susceptibility to disorders associated with both behavioral and inflammatory components and potentially alter their natural history. These concepts suggest that neuropharmacologic agents that stimulate hypothalamic CRH might potentially be adjunctive therapy for illnesses traditionally viewed as inflammatory or autoimmune.