OBJECTIVE: Erythrocytes (red blood cells [RBCs]) reduce extracellular ferricyanide by transmembrane transfer of reducing equivalents involving ascorbate recycling. RESEARCH DESIGN AND METHODS: Because ascorbate regeneration is glutathione (GSH) dependent and cells may be depleted of GSH in diabetes, we measured RBC GSH, plasma sulfhydryl (SH) groups, and RBC-mediated ferricyanide reduction in 30 type 1 diabetic patients (age 34 +/- 10 years, disease duration 20 +/- 8 years; no complications, n = 10; retinopathy, n = 10; nephropathy, n = 10), their 36 siblings (age 39 +/- 13 years), and matched healthy volunteers. RESULTS: Fasting plasma glucose was 15 +/- 7 mmol/l (vs. 5 +/- 1 in control subjects, P < 0.001), HbA1c 8.4 +/- 1.5% (vs. 5.4 +/- 0.3, P < 0.001), GSH 0.76 +/- 0.12 mg/ml packed RBCs (vs. 0.88 +/- 0.18, P < 0.01), SH groups 401 +/- 72 micromol/l (vs. 444 +/- 56, P < 0.05), and ferrocyanide generation 15 +/- 5 micromol/ml RBC per h (vs. 13 +/- 5, NS). In comparison with 10 normoalbuminuric diabetic subjects with retinopathy, 10 patients with diabetic nephropathy had similar fasting plasma glucose, HbA1c, and SH groups; lower RBC GSH (0.73 +/- 0.08 vs. 0.85 +/- 0.11, P < 0.05); and higher ferrocyanide generation (18 +/- 4 vs. 14 +/- 5, P < 0.05). The 10 patients without complications differed from the 10 healthy volunteers in glycemic control and RBC GSH. RBC electron transfer correlated with plasma lactate (r = 0.8, P = 0.01) only in the uncomplicated group. No difference was detected between siblings and healthy control subjects or between siblings of subjects in the nephropathy and retinopathy groups. Among diabetic patients, the rate of ferrocyanide generation was associated with urinary albumin excretion, plasma creatinine, and SH groups (multiple r = 0.6, P < 0.01). CONCLUSIONS: Transmembrane electron transfer is selectively increased in diabetic nephropathy, where RBC GSH is also depleted. The abnormality is peculiar to the nephropathy group and not contributed by familial or hereditary components because the electron flow was normal in siblings. The close relationship between cytosolic NADH and RBC electron transfer observed in diabetic patients without complications seems to be lost in the microangiopathic patients. Whereas patients with retinopathy alone still had normal activity of the RBC-reducing system, patients with nephropathy showed significantly increased activity, unrelated to metabolic parameters or plasma lactate concentration and correlated with renal function parameters and plasma thiols.