The endothelium plays a key role in the control of vascular patency and tone. Thus, the main objective of the study was to determine the role of endothelium and its derived relaxation factors in mediating relaxation of rat thoracic aorta, in response to sulfur dioxide (SO₂) derivatives “1:3 M/M sodium bisulfite (NaHSO₃) and sodium sulfite (Na₂SO₃)” using PowerLab tissue bath system. Endothelial denudation enhanced relaxation responses of SO₂ derivatives with an IC₅₀ of 6.11 mM as compared to control rings with an IC₅₀ of 6.21 mM, as well as the maximum relaxation (E_max) was increased from 62.026% ± 6.527% to 83.13% ± 14.755%. Furthermore, the relaxation responses to SO₂ derivatives in aortic rings were significantly enhanced by indomethacin, clotrimazole and methylene blue with IC₅₀’s of 4.8 mM, 5.33 mM and 4.01 mM, and E_max were raised to 101.1% ± 6.537%, 66.92 ± 7.538 and 104.68 ± 3.575, respectively. Meanwhile, L-NAME did not alter dose-dependent relaxation of SO₂ derivatives in comparison to control aortic rings. The results of this study had shown that endothelium denudation and blocking of endothelium derived-relaxation factors enhanced vasodilator effect of SO₂; this may clarify the role of endothelium in the vasodilatory mechanism of SO₂.