Replication-defective mutants of herpes simplex virus type 1 (HSV-1) are powerful tools to transfer genes into postmitotic neurons and show promise for gene therapy protocols in vivo. To evaluate the efficacy and safety of these vectors for the treatment of deafness we infected dissociated cochlear ganglia with HSV mutants defective in the immediate early genes IE 2 (5dl1.2) or IE 3 (d120). Our results reveal striking differences in the survival of neuronal and non-neuronal cells caused by these mutants. Surprisingly, cochlear neurons infected with 5dl1.2 at various concentrations show a significant increase in survival after 2 days in culture. In contrast, many non-neuronal cells undergo apoptosis reducing cell number to less than 50%. In both neuronal and non-neuronal cell types we also observe a population of cells with important changes in morphology. Analysis of dissociated cochlear ganglia infected with d120 reveals a decrease of neuronal survival, whereas non-neuronal cells were almost unaffected. To further characterize and compare the effects of 5dl1.2 and d120 we transduced central nervous system-derived cell types including cortical neurons and astrocytes. Similarly, as observed for cochlear neurons, infection with 5dl1.2 results in increased survival of cortical neurons, whereas d120 shows cytotoxic effects. Survival of astrocytes is equally reduced by both HSV deletion mutants. We conclude that HSV-1 mutants defective in immediate early genes cause very distinct cytopathic phenotypes depending on the cellular context. Possible reasons for these differences, like various patterns of cellular and viral gene expression, and the implications for the use of HSV-1 vectors for gene transfer are discussed.