Following inoculation into the snout herpes simplex virus (HSV) spread to neurons in mouse trigeminal ganglion and subsequently to the brain. Capsaicin treatment of neonatal mice, which causes a loss of unmyelinated sensory neurons, some of which contain substance P, reduced the mortality rate of HSV-infected mice. Moreover, a lower percentage of mice survived the infection with reactivatable virus. There was also an extensive infection of glial cells proximal to the transitional zone in the trigeminal root between the peripheral and central nervous system. Distal to this zone there was an accumulation of substance P immunoreactivity in centrally directed fibres. This amplified degenerative effect on central branches of the substance P containing sensory nerves by glial infection may contribute to the deafferentiation pain syndrome following HSV infection.