The Semliki Forest virus (SFV) system has been shown to be highly efficient in transduction of cell lines and primary cells. We employed a novel "noncytotoxic" SFV(PD) vector for transduction of primary ventral midbrain floor cultures in vitro and rat substantia nigra in vivo. Rapid protein expression was noted with preferential transduction of neuronal cells including the dopaminergic subpopulation. To examine the suitability of the SFV vector system for functional gene expression, SFV(PD) vectors encoding for antiapoptotic proteins Bcl-X(L) and XIAP were designed. Despite effective transgene expression, SFV(PD) vectors were unable to rescue dopaminergic neurons from MPP+-induced apoptosis. In vivo, virus injection into substantia nigra resulted in fast onset of transgene expression, but elicited an activation of microglia and an inflammation response. We conclude that the use of novel SFV(PD) vectors is currently limited by persistent neurotoxicity of the vector system. Although SFV(PD) vectors may be useful for protein localization studies in dopaminergic neurons, functional applications will require the development of even less cytopathic vector systems.