Cell cycle progression represents a key event in vascular proliferative diseases, one that depends on an increased rate of protein synthesis. An increase in phosphatidylinositol 3-kinase (PI 3-kinase) activity is associated with vascular smooth muscle cell proliferation, and rapamycin, which blocks the activity of the mammalian target of rapamycin, inhibits this proliferation in vitro and in vivo. We hypothesized that these 2 molecules converge on a critical pathway of translational regulation that is essential for successful upregulation of cell cycle-regulatory proteins in activated smooth muscle cells. p70(S6) kinase, a target of PI 3-kinase and the mammalian target of rapamycin, was rapidly activated on growth factor stimulation of quiescent coronary artery smooth muscle cells and after balloon injury of rat carotid arteries. The translational repressor protein 4E-binding protein 1 was similarly hyperphosphorylated under these conditions. These events were associated with increases in the protein levels of cyclin B1, cyclin D1, cyclin E, cyclin-dependent kinase 1, cyclin-dependent kinase 2, proliferating cell nuclear antigen, and p21(Cip1) in vivo and in vitro, whereas inhibition of the PI 3-kinase signaling pathway with either rapamycin or wortmannin blocked the upregulation of these cell cycle proteins, but not mRNA, and arrested the cells in vitro before S phase. In contrast to findings in other cell types, growth factor- or balloon injury-induced downregulation of the cell cycle inhibitor p27(Kip1) was not affected by rapamycin treatment. These data suggest that cell cycle progression in vascular cells in vitro and in vivo depends on the integrity of the PI 3-kinase signaling pathway in allowing posttranscriptional accumulation of cell cycle proteins.