Recent data support the hypothesis that reactive oxygen species (ROS) play a central role in initiation and progression of vascular diseases. An important vasoprotective function related to regulation of ROS levels appears to be the antioxidant capacity of nitric oxide (NO). We previously reported that treatment with NO decreases phosphotyrosine levels of adapter protein p130cas, via increased protein tyrosine phosphatase PEST (PTP-PEST) activity, leading to suppression of agonist-induced hydrogen peroxide (H₂O₂) elevation and motility in cultured rat aortic smooth muscle cells. The current study was performed to investigate the hypotheses that insulin-like growth factor-1 (IGF-1) increases the activity of small GTPase Rac1 as well as H₂O₂ levels and that NO suppresses IGF-1-induced H₂O₂ elevation by decreasing Rac1 activity, via increased PTP-PEST activity and dephosphorylation of p130cas. We report that IGF-1 induces phosphorylation of p130cas and activation of Rac1 and that NO attenuates these effects. The effects of NO are mimicked by overexpression of PTP-PEST or dominant negative p130cas and antagonized by expression of dominant negative PTP-PEST or p130cas. We conclude that IGF 1 induces rat aortic smooth muscle cell motility by increasing phosphotyrosine levels of p130cas and activating Rac1, and that NO decreases motility by activating PTP-PEST, inducing dephosphorylation of p130cas and decreasing Rac1 activity. The latter effect then decreases intracellular H₂O₂ levels, thus attenuating cell motility.