Increased flux of glucose through the polyol pathway may cause generation of excess reactive oxygen species (ROS), leading to tissue damage. Abnormalities in expression of enzymes that protect against oxidant damage may accentuate the oxidative injury. The expression of catalase (CAT), CuZn superoxide-dismutase (CuZnSOD), glutathione peroxidase (GPX), and Mn superoxide-dismutase (MnSOD) mRNA was quantified in peripheral blood mononuclear cells-obtained from 26 patients with type 1 diabetes and nephropathy, 15 with no microvascular complications after 20 years' duration of diabetes, and 10 normal healthy control subjects-that were exposed in vitro to hyperglycemia (HG) (31 mmol/l D-glucose). Under HG, there was a twofold increase in the expression of CAT, CuZnSOD, and GPX mRNA in the patients without complications and the control subjects versus patients with nephropathy (P < 0.0001), and MnSOD did not change in any of the groups. The aldose reductase inhibitor zopolrestat partially restored the levels of CAT, CuZnSOD, and GPX mRNA in the patients with nephropathy (P < 0.05). There was a highly significant correlation between increased aldose reductase (ALR2) expression, CAT, CuZnSOD, and GPX mRNA levels under HG conditions and polymorphisms of ALR2 in the patients with nephropathy (P < 0.00001). In conclusion, these results suggest that high glucose flux through aldose reductase inhibits the expression of antioxidant enzymes.