Stimulation of rat vascular smooth muscle cells (VSMCs) by platelet-derived growth factor (PDGF) transiently increased the intracellular concentration of hydrogen peroxide (H2O2). This increase could be blunted by increasing the intracellular concentration of the scavenging enzyme catalase or by the chemical antioxidant N-acetylcysteine. The response of VSMCs to PDGF, which includes tyrosine phosphorylation, mitogen-activated protein kinase stimulation, DNA synthesis, and chemotaxis, was inhibited when the growth factor-stimulated rise in H2O2 concentration was blocked. These results suggest that H2O2 may act as a signal-transducing molecule, and they suggest a potential mechanism for the cardioprotective effects of antioxidants.