Diabetes mellitus (DM) is a chronic metabolic disorder considered to be a major risk factor for cardiovascular diseases. DM-induced endothelial dysfunction is associated with cardiovascular risk. In human and diabetic animals models, decrease levels of glycosaminoglycans (GAGs) have been associated with a generalized vascular dysfunction (Deckert y col. 1989), and recent studies has demonstrated a beneficial effect of sulodexide (SUL) (glycosaminoglycan composed from heparin-like and dermatan fractions) in experimental models of endothelial damage (Kristová et al. 2000; 2008).Since NO plays a key role in vascular function, we assessed the possible endothelium-protective effect of SUL (15mg/kg, s.c) on vascular relaxation and nitric oxide synthase (NOS) activity in aortic rings of diabetic rats. Diabetes type 1 was induced in male Sprague-Dawley rats (200-250, n=8-10/group) by administration of streptozotocin (STZ), 60 mg, i.v. Animals were randomly allocated in four groups: C=control, STZ=diabetic, SUL=control+SUL and STZ+SUL= diabetic+SUL. After three months of treatment, the animals were euthanized in thiopenthal anesthesia (30 mg/Kg i.p.) and the aorta was microdissected. Basal and SUL-stimulated NOS activity was assayed by monitoring the conversion of [3H]-L-arginine to [3H]-L-citruline. Phenilephrine precontracted aortic rings were used to evaluated acetylcholine (ACh: 1×10-7, 3×10-7, 1×10-6, 3×10-6) and nitroprusiate (NPS:10-8,10-7,10-6) relaxation capacity. Values are means ± E.E.M, compared by ANOVA. Basal NOS activity was lower in STZ-diabetic rats than in control group (57.2±4.02 vs. 90.58±4.6 p< 0.01), and this activity was restored by SUL treatment (100.83±12). In diabetic rats ACh relaxation was 28.8-35.1% lower than in control rats, but relaxation in SUL y SUL+STZ groups was similar to control. No significative statistical differences were found in endothelium-independent NPS relaxation, between the different groups. Our results demonstrated that pretreatment with SUL prevents loss of the relaxing capacity induced by diabetes in aorta of diabetic rats, by a mechanism involving the production of nitric oxide, and suggest a potential use of this drug to improve endothelial dysfunction in diabetes.