We developed a defective herpes simplex virus (HSV) vector system that permitted the introduction of virtually any gene into mammalian central nervous system neurons. The HSV-1 amplicon plasmid, pHSL, contained a beta-galactosidase transcription cassette directed by HSV-1 IE68 promoter and two elements from HSV-1 genome: the origin of HSV-1 replication and its packaging site sequence. Being transfected into immortalized cells such as Vero cells in the present of HSV-1 to provide the helper virus function, (in our tests, a temperature-sensitive mutank tsK strain with permissive temperature at 31 degrees C was used), the prototype vector pHSL could be packaged into HSV-1 particles as a head-to-tail concatemer with the size of about the genome of HSV-1. A mixed HSV-1 stocks obtained were called dvHSL. Infection of cultured embryonic spinal cord motor neurons and forebrain cortex neurons of rats with dvHSL showed that beta-galactosidase was expressed up to twelve days. Via corneal inoculation, the trigeminal ganglia of rats were infected by dvHSL and the expression of beta-galactosidase could be detected there at different times after inoculation for at least two months. Infection of the forebrain cortex of rats with dvHSL through stereotactic injection showed that the expression of beta-galactosidase was restricted to the injection site for up to two months.