The expression of the immunosuppressive cytokine interleukin (IL)-10 in the normal and Toxoplasma gondii-infected murine brain was analysed. Microglia/macrophages expressed IL-10 at the mRNA and protein level in the normal brain. In Toxoplasma encephalitis (TE), CD4+ and CD8+ T-cells also contributed to the upregulated IL-10 production. Neutralization of endogenous IL-10 in chronic TE reduced the intracerebral parasitic load and increased the number of immune cells and the production of protective cytokines. These findings indicate that intracerebral expression of IL-10 interferes with the immune response in TE and may contribute to parasite persistence in the brain.