Our previous studies have proven that crocetin (CCT), extracted from Gardenia jasminoides Ellis, possesses the anti-atherosclerotic effect. Because endothelial dysfunction strongly contributes to the initiation and progression of atherosclerosis, the present study aims to investigate whether CCT is capable of improving this dysfunction and to explore the possible mechanisms. Endothelial dysfunction was induced by in vivo feeding high cholesterol diet (HCD) to rabbit and by in vitro treating bovine aortic endothelial cells (BAECs) with oxidized LDL (oxLDL). Endothelium-dependent relaxation (EDR) evoked by acetylcholine (Ach) and endothelium-independent relaxation (RIDR) mediated by sodium nitroprusside (SNP) of thoracic aorta isolated from rabbit were measured. The results indicated that the EDR in HCD alone treated rabbits was seriously impaired and the maximal relaxation induced by Ach (10(-5.5) M) was only 54% that in control rabbit fed with regular diet. Oral complementation with CCT (15, 30 mg/kg) dose-dependently improved this impairment and restored the maximal relaxation to 68% and 80% that in control group, respectively. However, the EIDR maintained comparable in all groups. Complementation with CCT (15, 30 mg/kg) simultaneously increased serum level of nitric oxide (NO), upregulated vessel activity and mRNA expression of endothelial NO synthase (eNOS) as well as vessel cyclic GMP (cGMP) content compared with those in rabbit treated with HCD alone. Inducible NOS (iNOS) activity remained unchangeable in all groups. In BAECs, oxLDL treatment decreased NO production, downregulated both activity and mRNA expression of eNOS. While those decrease or downregulation were inhibited by co-treatment with CCT (0.1, 1, 10 microM) in a dose-dependent manner. These findings suggested that CCT significantly restored the EDR of thoracic aorta in hypercholesterolemic rabbit, which might be explained by its action to increase the vessel eNOS activity, leading to elevation of NO production.