Abstract: Experiments were performed to elucidate the mechanism of action of a 7-day oral administration of the sulfur-containing angiotensin-converting enzyme (ACE) inhibitor 3-thienylalanine-ornithyl-proline (TOP; 10 mg/kg/d) on endothelial dysfunction and oxidative stress compared with that of captopril (control; 40 mg/kg/d) in spontaneously hypertensive rats. The differential expression of messenger RNA by real-time reverse-transcriptase–polymerase chain reaction and protein by Western blot analysis was assessed for the markers nicotinamide adenine dinucleotide phosphate oxidase, p22phox, endothelial nitric oxide (NO) synthase, and AT1 receptor. Furthermore, TOP-induced vascular relaxation was also investigated using rat aortic rings in an organ bath. TOP significantly downregulated both messenger RNA and protein expressions of p22phox and AT1 receptor; the latter facilitates vasoconstriction through angiotensin II. In addition, TOP upregulated endothelial NO synthase, thus enhancing the production of NO. Vascular studies revealed that TOP caused endothelium-dependent vasorelaxation. In conclusion, unlike the free sulfur in captopril, the thiophene ring in TOP may act as a better scavenger of free radicals. Therefore, TOP exerted more significant antihypertensive effects than captopril, not only through angiotensin-converting enzyme inhibition but also through more effective antioxidation, because the inherent thiophene moiety resulted in the enhanced production of NO.