Diabetes mellitus is a major risk factor for atherosclerosis. In population based studies diabetes is associated with an increased incidence of coronary heart disease [1], cerebrovascular disease and peripheral vascular disease [2]. The high prevalence of macrovascular disease is partly explained by the increased frequency of conventional risk factors and partly by risk factors specific for diabetes including hyperglycaemia, hyperinsulinaemia and dyslipidaemia. The view that diabetic vasculopathy is, at least in part, related to excess free radicals is supported by evidence such as increased lipid peroxidation [3] and reduced antioxidant reserve in patients with diabetes [4] and from studies showing a direct cytotoxic effect of free Diabetologia (1998) Summary Vascular smooth muscle cell (VSMC) dysfunction plays a role in diabetic macrovasculopathy and this may include abnormalities in growth characteristics and the extracellular matrix. As the actual mechanisms by which glucose induces VSMC dysfunction remain unclear, the aim of this study was to assess the potential role of glucose-induced oxidative stress. Porcine aortic VSMCs were cultured for 10 days in either 5 mmol/l normal glucose or 25 mmol/l D-glucose (high glucose). There was evidence of oxidative stress as indicated by a 50 % increase in intracellular malondialdehyde (p < 0.05), increased mRNA expression of CuZn superoxide dismutase and Mn superoxide dismutase (by 51 % and 37 % respectively, p < 0.01) and a 50 % decrease in glutathione in 25 mmol/l D-glucose (p < 0.001). Growth was increased by 25.0 % (p < 0.01). mRNA expression of extracellular matrix proteins (collagens I, III, IV and fibronectin) was not altered by high glucose in these experimental conditions. Repletion of glutathione with N-acetyl L-cysteine (1 mmol/l) in VSMC grown in high glucose was associated with reduction in malondialdehyde and restored growth to that of normal glucose. The water soluble analogue of vitamin E, Trolox (200 mmol/l), reduced malondialdehyde concentrations, but had no effect on glutathione depletion or the increased growth rate seen with high glucose. The addition of buthionine sulphoximine (10 mmol/l) to VSMC cultured in normal glucose reduced glutathione, increased malondialdehyde and increased growth to a similar extent as that found in high glucose alone. These results suggest that thiol status, rather than lipid peroxides, is a key factor in modulating VSMC growth and that mRNA expression of extracellular matrix proteins is not increased in VSMC under conditions of glucose-induced oxidative stress. [Diabetologia (1998