Background : While inositol phospholipid-specific phospholipase C (PLC) plays a central role in signal transduction pathways, little is known about its role in the vascular response to injury. Recent studies have shown that phospholipase C-γ1 (PLC-γ1) is required for PDGF-induced DNA synthesis and angiotensin II signaling. This study was undertaken to determine the potential involvement of PLC-γ1 in the in vivo response to vascular injury. Methods : Vascular injury was achieved in the left common carotid artery of six-month-old male Wistar rats. The expression of PLC-γ1 was evaluated at serial time points by immunohistochemistry and Western blot analysis following balloon de-endothelialization of the rat carotid artery. Results : In the denuded carotid artery at 1 week, the neointima became thicker in a symmetrical manner with respect to the long axis. A strong expression of PLC-γ1 at one week after injury was seen primarily in the thin layers of neointima. This increased immunoreactivity of PLC-γ1 persisted at 2-3 weeks after injury, coinciding with the time when neointima gains of its mass. At 4 weeks after injury, staining intensity slightly declined but levels remained elevated. As determined by Western blot analysis, the amount of PLC-γ1 was about 3-fold higher at 3 weeks after injury compared to uninjured vessels (p<0.01). Conclusion : These results suggest that the amplification of traffic within signal transduction pathways involving PLC-γ1 occurs and may play a significant role in neointima formation following arterial injury.(Korean J Med 60:234-241, 2001)