Immunoflourescnce and ultrastructure comparative studies were performed on brain tissues of mice experimentally infected with R.H. strain of T. gondii and those challenged post-vaccination. Immunoflourescence studies revealed that there were variable expressions of antigenic materials between the two groups. The brain tissue of infected mice showed positivily reacted numerous clusters of extra or intra-cellular tachyzoites within the cytoplasm of neurons or neuroglia cells meanwhile the brain tissue of  the vaccinated group, exhibited little expression of antigenic materials mainly of intracellular location with less evidence of extracellular ones and the intensity of the staining reaction was relatively less than that of infected ones. Ultrastructure studies of brain tissue of infected mice showed one or two intracellular tachyzoites within the cytoplasm of neurons or neurologia cells particularly in astrocytes. The intracellular tachyzoites were surrounded by electron-lucent parasitophorous vacuole. The host cells revealed collection of numerous mitochondrial elements around the limiting membrane of the parasitophorous vacuole meanwhile, other mitochondria appeared to be degenerated and/or necrotic with loss of its cristea. The nuclei of parasitized cells depicted marginated nuclear heterochromatin, wrinkled nuclear envelope or complete lysis of the cytoplasmic and nuclear components. The brain tissue of vaccinated group showed different morphological deformities of the intracellular tachyzoites represented by irregular contours of the parasite pellicle, constriction of its anterior conoidal end, pyknosis of its nuclei and fragmentation of its cellular constituents into more electron dense bodies. Moreover, some parasitized cells relatively maintained the fine structure of its cytoplasmic organelles, with significant irregularities of the nuclear envelope into series of concavities, meanwhile few cells appeared disintegrated with fragmentation of its cellular constituents. There were little evidence of multiplication of intracellular tachyzoites in the vaccinated group.