To assess in vivo effects of antioxidants on vascular cell adhesion molecule (VCAM)-1 expression, circulating soluble VCAM-1 and intraerythrocytic reduced glutathione (GSH) and GSH disulphide (GSSG) concentrations were evaluated in non-insulin-dependent diabetic patients without complications (9 men, 6 women, 48 ± 6 years old) before and after 1 month of either oral N-acetyl-L-cysteine (1.200 mg/day) or placebo treatments, given in randomized, cross-over, double-blind fashion. Ten healthy subjects (7 men, 3 women, 52 ± 4 years old) served as control subjects. Baseline plasma VCAM-1 concentrations were higher (p = 0.007) in non-insulin-dependent diabetic patients (707.9 ± 52.5 ng/ml) than in control subjects (627.3 ± 84.6 ng/ml). Intraerythrocytic GSSG content was higher (non-insulin dependent diabetic patients: 0.618 ± 0.185 μmol/g Hb; control subjects: 0.352 ± 0.04 μmol/g Hb, p = 0.0002), whereas intraerythrocytic GSH concentrations were lower (p = 0.001) in non-insulin dependent diabetic patients (6.0 ± 0.7 μmol/g Hb) than in control subjects (7.1 ± 0.5 μmol/g Hb). The mean GSH:GSSG ratio was also lower (p = 0.0001) in the first (10.9 ± 4.5) than in the second group (20.2 ± 1.4). Circulating VCAM-1 and intraerythrocytic GSH concentrations were negatively correlated in non-insulin diabetic patients (r = 0.605, p = 0.01). Treatment with N-acetyl-L-cysteine decreased plasma VCAM-1 (p = 0.01) and intraerythrocytic GSSG (p = 0.006) but increased GSH concentrations (p = 0.04) and the GSH:GSSG ratio (p = 0.004) in non-insulin dependent diabetic patients. Our data indicate that the vascular endothelium is activated in non-insulin dependent diabetes. Antioxidant treatment counterbalanced such endothelial activation. Thus, antioxidant agents might protect against oxidant-related upregulation of endothelial adhesion molecules and slow down the progression of vascular damage in non-insulin dependent diabetes. [Diabetologia (1998) 41: 1392–1396]