Diabetes mellitus is associated with many complications including retinopathy, nephropathy, neuropathy and angiopathy. Increased cardiovascular risk is accompanied with diabetes-induced endothelial dysfunction. Pharmacological agents with endothelium-protective effects may decrease cardiovascular complications. In present study sulodexide (glycosaminoglycans composed from heparin-like and dermatan fractions) was chosen to evaluate its protective properties on endothelial dysfunction in diabetes. Effect of sulodexide treatment (SLX, 100 UI/kg/day, i.p.) in 5 and 10 weeks lasting streptozotocin-induced diabetes (30 mg/kg/day, i.p. administered for three consecutive days) was investigated. Animals were divided into four groups: control (injected with saline solution), control-treated with sulodexide (SLX), diabetic (DM) and diabetic-treated with sulodexide (DM+SLX). The pre-prandial and postprandial plasma glucose levels, number of circulating endothelial cells (EC) and acetylcholine-induced relaxation of isolated aorta and mesenteric artery were evaluated. Streptozotocin elicited hyperglycemia irrespective of SLX treatment. Streptozotocin-induced diabetes enhanced the number of circulating endothelial cells compared to controls. SLX treatment decreased the number of EC in 10-week diabetes. Acetylcholine-induced relaxation of mesenteric arteries was significantly impaired in 5 and 10-week diabetes. SLX administration improved relaxation to acetylcholine in 5 and 10-week diabetes. Diabetes impaired acetylcholine-induced relaxation of rat aorta irrespective of SLX treatment. Our results demonstrate that SLX treatment lowers the number of circulating endothelial cells and improves endothelium-dependent relaxation in small arteries. These findings suggest endothelium-protective effect of sulodexide in streptozotocin-induced diabetes.