Latent herpes simplex virus (HSV) infection of the trigeminal ganglion of guinea pigs and latent varicella-zoster virus (VZV) infection of the trigeminal ganglion of humans were studied by in situ nucleic acid hybridization. Guinea pig trigeminal ganglia were removed during the period of viral latency (four to five weeks after corneal inoculation of HSV), and human ganglia were removed at autopsy. Radiolabeled HSV and VZV DNAs were used to probe ganglion tissue sections for viral-specified RNA. Hybridization detected only over neurons was present in 46 percent of ganglia from 22 latently infected guinea pigs and from 33 percent of ganglia from 10 human subjects. These results support the conclusion that some viral transcription occurred during HSV and VZV latency.