Proinflammatory cytokines [interleukin (IL)-1 and -6 and tumor necrosis factor-] function within a complex network, stimulating the release of one another, as well as other cytokine agonists and antagonists. These interactions have not been as widely studied in vivo. Therefore, the following studies measured cytokines in blood and cerebrospinal fluid (CSF) from juvenile rhesus monkeys after intravenous administration of cytokines. IL-1 and IL-1 were equally effective in elevating blood levels of IL-6. In contrast, IL-1 was the only cytokine that significantly elevated IL-6 levels in the CSF. Interestingly, both IL-1 and IL-6 increased levels of IL-1 receptor antagonist in the blood and comparably stimulated the release of cortisol. A second study confirmed that the IL-1-induced IL-6 in CSF was brain derived and not a result of diffusion from blood. This research extends studies of the cytokine cascade to the central nervous system (CNS), highlighting the brain response to peripheral activation.