BACKGROUND: The granulocyte colony-stimulating factor (G-CSF) is crucial in neutrophil regulation. Since recombinant G-CSF became clinically available, it has been widely used in the treatment of neutropenia. Ex vivo therapy of recombinant G-CSF, however, requires large dose and frequent administration, which brings financial burden on the patients. To overcome disadvantages of ex vivo therapy, we have tried to make an in vivo G-CSF delivery system in rat using gene therapy technique. METHODS: We have tried to make an in vivo G-CSF delivery system using transduced vascular smooth muscle cells with G-CSF gene in a rat model. Retroviral vector plasmid containing rat G-CSF gene was made employing LXSN and LNFX plasmid. Recombinant retrovirus was produced from PA317 packaging cells. Infection of the vascular smooth muscle cells with the virus and selection with G418 was done in vitro. These transduced cells were transplanted to the balloon-injured carotid arteries of Fisher 344 rats, and complete blood count as well as differentials were measured in sequence. RESULTS: The virus titer was three times greater in case of LNFG than LGSN, whereas G-CSF production from infected vascular smooth muscle cell was lower in LNFG vector (0.1ng/106cells/day) than in LGSN vector (0.4ng/106cells/day). The increment of WBC count was observed until 25 days after transplantation, being 9,600 +/- 1,000/uL on seventh day after transplantation, which was significantly higher than that of controls, 7,300 +/- 540/uL. The levels of neutrophil increased gradually after transplantation, reached to the peak after 1 week (3,250 +/- 1,099/uL in case of neutrophil count and 30 +/- 10% in case of differentials). The duration of increment, however, was relatively short, neutrophil count being decreased to the basal level within 4 weeks. CONCLUSION: The effective increase of neutrophil count with low dose of G-CSF produced from vascular smooth muscle cells could make this gene therapy feasible in the clinical settings only if the problem of short duration of effect could be solved.