The peripheral inoculation of herpes simplex virus type 1 (HSV 1) in experimental animals induces central nervous system (CNS) demyelinating lesions, but the potential relevance of this model to multiple sclerosis is lessened by the unifocal nature of the lesion. In this study, inbred strains of mice were selected on the basis of varying resistance to mortality following lip inoculation with virus. A spectrum of CNS pathology was observed, ranging from focal collections of inflammatory cells at the trigeminal root entry zone in resistant strains (C57BL/6J), to unifocal demyelinating lesions in moderately resistant strains (BALB/cByJ), to multifocal demyelinating lesions throughout the brain in susceptible strains (A/J). Findings from viral titration studies of the CNS support a direct cytolytic effect of virus in the development of demyelinating lesions at the trigeminal root entry zone but cannot exclude an immune-mediated component. Furthermore, 50% tissue-culture-infective doses, immunofluorescence, and electron microscopic studies of primary cultures of oligodendrocytes, derived from the three strains of adult mice, identify differences in resistance to HSV 1 infection in vitro, suggesting that differences at this level may also contribute to the pathological appearance. Multifocal lesions in A/J mice were first observed when the infectious virus could no longer be isolated from the CNS and may be the result of an immune-mediated process "triggered" by the acute CNS infection in susceptible strains of mice.