To study early events of neonatal herpes simplex virus (HSV) encephalitis and its sequelae, the authors induced a controlled infection in the brains of mice using HSVgH, a genetically modified Disabled Infective Single Cycle virus. Neonatal Balb/C mice were infected with various amounts of HSVgH- virus by intracerebral injection. Results showed that the survival of infected mice was dependent on the amount of virus injected. Infection with 200,000 plaque forming units (pfu) of HSVgH-, virus resulted in 0% survival, whereas 25,000 pfu resulted in 75% survival. If the mice died, 98% of the deaths occurred between 3 and 7 days after infection. Replication competent virus was recovered from 20% of mice brains infected with 25,000 pfu of HSVgH-. Neutralizing antibodies were not detected 6 weeks post infection in sera of mice, which survived infection with 25,000 pfu of HSVgH-. LacZ histochemistry and immunoperoxidase staining using anti-HSV and anti- beta-galactosidase antibodies revealed that the infection was limited to the site of injection. Tissue destruction was observed at the site of inoculation 3 days post infection using cresyl violet staining. At 3 days post infection adjacent sections showed positive cells for viral antigens and apoptotic cells in the infected area. Immunoperoxidase staining using antibodies to surface markers showed microglial activation beginning on day 1 and astrocyte proliferation beginning on day 3 post infection. B and T lymphocytes were not detected on day 1 through 7 post infection. This controlled experimental HSV infection suggests a limited non-specific early host response in the neonate to HSV encephalitis.