Objectives: The aim of the present study was to evaluate the influence of endothelial nitric oxide (NO) on the vascular responses of curcumin-treated rats. Methods: The experimental groups included the control and curcumin-treated (200 mg/kg/day, p.o., for 4 weeks) group. The concentration response curves to receptor-dependent agent 5-hydroxytryptamine(5-HT; 10-9-3×10-4 M) and receptor-independent agent potassium chloride (KCl; 5-100 mM) were observed. Results: The concentration response curves to 5-HT and KCl shifted to the right and the maximal response was significantly decreased in the curcumin-treated rat aortas. A pretreatment of rings with L-NAME (a NOS inhibitor, 10-4 M) increased both the sensitivity and maximal response to only 5-HT. No apparent histological changes were demonstrated in smooth muscle and connective tissue layers in the aortas of the control and curcumin-treated rat preparations. Conclusion: The results of the present study suggest that NO release from endothelial cells modulates curcumin- treated rat aorta responses to 5-HT, but not to KCl.