Acute lethal infections were obtained in mice by intraperitoneal (IP) injection of 10(2) or 10(4) tachyzoites of the virulent RH and C56 strains. Chronic infections were obtained by IP injection or peroral (PO) gavage of 20 cysts of the avirulent C strain. Mice were sacrificed at varying intervals after infection and parasite burdens were quantitated in blood, brain, and lungs using a tissue culture method. Acutely infected mice died within 6 to 10 days postinfection as a function of the strain and inoculum size. With either strain, tachyzoites were first detected in lungs on either Days 2 or 4 postinfection, according to the inoculum size, then in brain and blood at Days 4 or 6; parasitic loads remained constantly at a higher level in lungs than in brain until the date of death. Bradyzoites could only be detected in lungs, from Days 4 or 6 until death. In chronic infections, similar results were obtained for IP and PO infected mice. Both tachyzoites and bradyzoites were first detected in lungs and brain from Day 7 after infection; tachyzoites remained at a higher level in lungs than in brain until Day 10, then subsequently decreased in lungs. At Day 50, tachyzoites were not detectable in lungs, whereas bradyzoites remained at a constant level; in brain, both parasitic stages were detectable at a similar level throughout the follow-up period. These results indicate that infection with a virulent Toxoplasma strain is characterized by an early involvement of lungs, with pneumonia as the principal cause of death.(ABSTRACT TRUNCATED AT 250 WORDS)