An emerging literature attests to the ability of psychological stress to alter the inflammatory cytokine environment of the body. While the ability of stress to cause cytokine release is well established, the neural pathways involved in this control have yet to be identified. This study tests the hypothesis that IL-6 neurons of the hypothalamo-neurohypophyseal system (HNS), a neural pathway proposed to secrete IL-6 into the circulation, are activated in response to psychological stress. Colocalization studies confirm robust expression of IL-6 in cell bodies and fibers of vasopressin (but not oxytocin) neurons of the paraventricular (PVN) and supraoptic nucleus (SON) of the rat hypothalamus. In response to restraint, there was a greater increase in c-Fos expression in SON IL-6-positive (IL-6ϩ) neurons. In addition, both psychogenic (restraint) or systemic stress (hypoxia) lead to phosphorylated ERK induction only in IL-6ϩ magnocellular neurons, indicating selective activation of the MAPK signaling pathway in the IL-6 subset of magnocellular neurons. Finally, restraint upregulated IL-6 mRNA expression in both the PVN and SON, which was accompanied by a four-fold increase in circulating IL-6. The data indicate that noninflammatory stressors selectively activate IL-6 magnocellular neurons, upregulate IL-6 gene expression in the PVN and SON, and increase plasma IL-6. In summary, results show that IL-6 neurons of the HNS are a recruited component of the response to psychological stress. cytokine; magnocellular; supraoptic nucleus; paraventricular nucleus; hypothalamo-neurohypophyseal system STRESS CAUSES THE RELEASE of proinflammatory cytokines. The impact of stress on the inflammatory environment is a key mechanism by which stress negatively affects health (18). Inflammation is a contributing factor in a variety of human diseases (10) and is implicated in the progression of metabolic disorders (4), cardiovascular disease (2), depression (27), and autoimmune disease (9), all disorders that are stress sensitive.The acute-phase response to psychological stress is quite similar to the acute-phase response to infection, even when the perceived stressor poses no immediate threat to homeostasis. Like infection, stress increases proinflammatory cytokines (31), body temperature (22), activation of the hypothalamopituitary-adrenocortical (HPA) axis, and production of acutephase proteins in the liver (13). Although the magnitude of change in individual factors may vary, there is a large overlap between responses to stress and infection. With infection, IL-1, IL-6, and TNF-␣ are the primary mediators of the systemic acute-phase response and are derived from immune cells. In contrast, immune cells collected after psychological stress in humans do not show increased in vitro cytokine production (31), despite the fact that there are increases in the circulating concentration of IL-1 and IL-6. In particular, IL-6 production from immune cells collected in vivo then stimulated in vitro does not increase following stress exposure (31...