C57BL/10 ScSn (B10) mice infected orally with Toxoplasma gondii were killed on days 5, 10, 15, 20 and 30 post infection and their brains excised. These were either used to count total tissue cyst numbers or divided for RNA purification and histopathological studies. The first signs of inflammation were on day 10 post infection, before the appearance of cysts in the brain, and correlating with the appearance of activated astrocytes. These mice had a mild meningitis with areas of encephalitis. Small numbers of cyst stages were first observed in the brain on day 15 and by day 20 the cyst numbers had increased dramatically but were not always associated with inflammation. After this time point, total cyst numbers did not increase significantly though there developed a marked variation in tissue cyst size with larger cysts becoming more numerous. The use of the polymerase chain reaction to assist in the amplification of brain RNA allowed the characterization of the kinetics of cytokine production within the brains of these animals. Only IL-1α was found to be expressed constitutively in control mice. Transcripts for other cytokines associated with activated monocytes, microglial cells and astrocytes [tumor necrosis factor (TNF)-α and interleukin (IL)-6] were present on day 10, (IL-6) and day 15 (TNF-α) post infection. Thereafter, these cytokines were present in all infected animals. Of the T cell-associated cytokines, IL-4, a characteristic product of the T helper (Th)2 cell subset, was detected on days 10 and 15, while granulocyte macrophage colony stimulating factor (GM-CSF) which can be produced not only by this cell type but also by Th1 cells and CD8+ T cells, was also present on day 15 but not thereafter. Transcripts for interferon (IFN)-γ, present from day 15 post infection, were probably produced by CD8+ T cells, as IL-2 which would indicate Th1 cell involvement was only detected 30 days after infection. The continual presence of IFN-γ and TNF-α, cytokines with reported anti-toxoplasmic activity, in the CNS of B10 mice throughout the latter half of the experimental period did not diminish the severity of infection. These results indicate that the CD4+ Th2 subset may allow a rapid rise in cyst numbers and so be important in determining susceptibility to toxoplasmic encephalitis.