The brain's response to a direct immune challenge was examined by in situ hybridization histochemistry. Lipopolysaccharide (bacterial endotoxin) injected acutely into rat striatum induced mRNA expression for inhibitory factor kappaBalpha, interleukin (IL)-1beta, tumor necrosis factor-alpha, IL-6, IL-12 p35, inducible nitric oxide synthase, IL-1 receptor antagonist, and the type 1 IL-1 receptor. Expression patterns were evaluated at select time points ranging from 15 min to 3 days post-injection. Rats injected with vehicle alone were used to control for mechanical effects. Following lipopolysaccharide administration, a wave of mRNA induction within brain parenchyma radiated outward from the injection site, generally peaking in intensity at the 16-h time point. The individual profiles of cytokine mRNA induction patterns reveal that the brain's immune response to local inflammatory stimulation is quite elaborate and in many ways resembles the progression of cytokine induction customary of localized inflammation in peripheral tissues.