Proliferation and migration of vascular smooth-muscle cells (VSMCs) are essential events in neointimal hyperplasia. Recent findings that active oxygen species induce pro-oncogene expression, and stimulate VSMC DNA synthesis and cell division, suggest that active oxygen species may play an important role in intimal proliferation after arterial injury. To determine how the redox state of the artery was altered by injury, the levels of thiobarbituric-acid-reactive substances (TBARs, markers of lipid oxidation) and glutathione peroxidase (GSH-PX, the enzyme responsible for the production of glutathione, a major intracellular antioxidant) were measured in the rat aorta after balloon injury. There was an inverse relationship between the level of TBARs, which increased significantly to a maximum 17% greater than normal at 10 days after injury (p < 0.05, n = 7), and the activity of GSH-PX, which decreased significantly to a minimum 36.5% less than normal at 10 days after injury (p < 0.05, n = 7). To determine whether maintaining a more reduced vessel environment would inhibit intimal proliferation, L-cysteine was administered by intraperitoneal injection from 3 days before to 14 days after balloon injury. At 14 days after the arterial injury, the aorta was harvested for histological and morphometric measurements of TBARs and GSH-PX. At 7 and 14 days after balloon injury, the aorta was harvested for [<sup>3</sup>H]thymidine incorporation studies. Efficacy of therapy was demonstrated by a significant decrease in the level of TBARs and an increase in the activity of GSH-PX (p < 0.05 vs. the control group, n = 7). In the L-cysteine group, all parameters of intimal proliferation were significantly reduced compared to the controls, including the intima:me-dia ratio (0.091 vs. 0.253, p < 0.05); the cross-sectional area of the neointima (0.0563 compared to 0.140 mm<sup>2</sup>, p < 0.05), coverage of the internal elastic lamina by the neointima (23.44 compared to 43.27%, p < 0.05), and [<sup>3</sup>H]thymidine incorporation (at 7 days 240.58 vs. 350.68 cpm/mg tissue, p < 0.05; at 14 days 181.71 vs. 275.30 cpm/mg tissue, p < 0.05). These results demonstrate dynamic alterations in the vessel redox state after arterial injury, and suggest that maintaining a more reduced environment (e.g. administration of L-cysteine) will reduce intimal proliferation after arterial injury.