To investigate the modulation of CO2 and endothelium of vascular contraction induced by various agonists, we studied the influence of high PCO2 (PCO2 = 91 mmHg, pH = 6.99) on the response of endothelium-intact and -rubbed rat aortic preparations to KCl, phenylephrine (PE), and human-porcine endothelin-1 (ET-1). Response of endothelium-intact aortic preparations to KCl was not influenced by both high PCO2 and the pH-matched acidotic solution (7.00) with normal PCO2, whereas that of endothelium-rubbed preparations was attenuated solely by high PCO2. With cyclooxygenase inhibitors or a thromboxane A2 receptor antagonist, high PCO2 attenuated the respose of both preparations to KCl. The dose-response curve of endothelium-intact and -rubbed preparations to PE was shifted to the right by both high PCO2 and the pH-matched acidotic solution with normal PCO2. The maximal response of endothelium-intact preparation to PE was attenuated by high PCO2. Indomethacin augmented the inhibitory action of high PCO2 on the PE-induced contraction. Contractile responses of endothelium-intact and -rubbed preparations to ET-1 were not influenced by high PCO2. With indomethacin, high PCO2 also had no influence on the ET-1-induced contraction of endothelium-intact preparations. Endothelium modified the high PCO2 effects on the time-contraction responses to the three agonists. CO2 and endothelium may variously modify the responses of rat aorta to different agonists. Cyclooxygenase-related eicosanoid(s) may be involved in the effects of high PCO2 on the response of rat aortic smooth muscle cells to KCl and PE.