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Parameters of virulence and the course of infection were compared in mice infected with Toxoplasma gondii strains P-Cz (mice virulent), 38 (cyst-forming, highly pathogenic) and HIF (cyst-forming, avirulent). Mice, that succumbed to infection with cystforming strain 38 had lesions in the spleen (days 7-14 p.i.), liver (days 7-14 p.i.), lungs (days 9-21 p.i.), heart (days 14-21 p.i.), and brain (days 21-35 p.i.). In contrast, the animals which succumbed to strain P-Cz had practically unaffected hearts and brains. There were no fundamental differences in the temporal sequence of histopathological changes in infections with virulent and both cyst-forming strains. Depending on the virulence of each strain the infections differed markedly in the degree of organ damage, duration of parasitemia, numbers of cysts in the brain, as well as in mortality and body weight loss. The differences in post mortem findings are not due to different organ affinity, but to the virulence-dependent time of death. All P-Cz infected mice die in the early stage of infection, before their hearts and brains can be seriously damaged.