Infection of the CNS by herpes simplex type 1 (HSV-1) via the trigeminal route to the brain stem was elucidated in a rat model. In contrast to the earlier described cortical and hippocampal infection after intracranial injection, the CNS showed a profound resistance to HSV-1 infection when the virus was administered by nose inoculation, as judged by histopathology and immunohistochemistry. In contrast, when the distribution of HSV-1 in the brain was investigated after nose inoculation by polymerase chain reaction, viral DNA was detected at all levels from the ganglia to the cortex. When replication of HSV-1 was assayed in primary cell cultures of rat astrocytes derived from brain stem, striatum, hippocampus and cerebral cortex, significantly lower virus yields were obtained in brain stem-derived astrocytes cultures as compared with in cortex-derived astrocytes. This finding was independent of whether HSV-1 strains used originated from brains of patients suffering from herpes simplex encephalitis or from patients with oral cutaneous lesions and lacking neurological symptoms. Also, by immunocytochemistry of cultures after HSV-1 infection, a lower number of plaques were seen in brain stem-derived astrocytes as compared with cortex-derived astrocytes. The observed relative resistance of brain stem-derived astrocytes to replicate HSV-1 might contribute to the ability of the brain stem to withstand infection during reactivation of this virus in the trigeminal neurons.