We have used a mouse model of herpes simplex encephalitis produced by intranasal inoculation of virus to study the expression of viral immediate early, early and late genes and latency associated transcript (LAT) in trigeminal ganglia and brain at various times after inoculation. A PCR technique was used to detect the viral gene transcripts. All viral genes were expressed between post-inoculation days 1 and 13. On post-inoculation day 42 when the acute infection had subsided only the LAT could be detected, most commonly (70%) in the trigeminal ganglion but also, in 50% of mice, in the brain stem, in 40% in olfactory bulbs and in 20% in cerebrum and cerebellum. These findings suggest that latent infection by HSV-1 may be relatively readily established in the CNS as well as in sensory ganglia. The frequency of establishment of latency appears to be related to the neuroanatomical accessibility of each brain region to the site of entry of the virus.