The contribution of atypical protein kinase C (PKC)-zeta to ANG II-accelerated restenosis after endoluminal vascular injury was investigated by using the rat carotid balloon injury model. Exposure of injured arteries to ANG II resulted in an extensive neointimal thickening (1.9 times) compared with vehicle at day 14. Treatment with PKC-zeta antisense, but not scrambled, oligonucleotides reduced neointimal formation observed in the presence or absence of ANG II. Examination of early events (2 days) after injury showed an increase in cellularity in the perivascular area of the artery wall that was transferred to the adventitia and media after exposure to ANG II, events blocked by PKC-zeta antisense, but not scrambled, oligonucleotides. A positive correlation between medial cellularity at day 2 and extent of neointimal growth at day 14 was established. Immunohistochemical analysis showed that upregulation of inflammatory markers after injury, as well as infiltration of ED1(+) monocytes/macrophages from the perivascular area to the adventitia, was accelerated by ANG II. However, ANG II-stimulated medial increase in cellularity was proliferation independent, and these cells were monocyte chemoattractant protein-1(+)/vimentin(+) but ED1(-)/VCAM(-). PKC-zeta is degraded after injury, and inhibition of its neosynthesis in medial vascular smooth muscle cells or in infiltrating cells with PKC-zeta antisense attenuated medial cellularity and expression of inflammation mediators without reversing smooth muscle cell dedifferentiation. Together, these data indicate that PKC-zeta plays a critical role in normal and ANG II-accelerated neointimal growth through a mechanism involving upregulation of inflammatory mediators, leading to cell infiltration in the media of the vascular wall.