The ability of the facultative intracellular pathogen Listeria monocytogenes to penetrate the central nervous system (CNS) was studied by following the kinetics of brain invasion and histological lesions during an acute intravenous (i.v.) infection in the mouse. CNS invasion occurred during the early phase of infection and produced severe meningoencephalitis characterized by multiple granulomatous foci predominantly located in the brainstem and associated with diffuse meningitis and an intense inflammatory reaction involving the choroid plexuses. Bacterial counts in the brain could reach 10(4.5)-10(5.8) by day 5 of infection with 1-2 x 10(6) bacteria i.v., depending upon the bacterial strain used. It was found that CNS invasion was highly dependent upon the level and the duration of bacteremia, thus indicating that persistent bacteremia is essential to induce meningoencephalitis to L. monocytogenes.