This study investigates the mechanism behind the decreased endothelium-dependent relaxations in the aorta of spontaneously hypertensive rats (SHR) compared to normotensive Wistar-Kyoto rats (WKY). Rings of thoracic aorta from both strains were isolated and suspended for isometric tension recording. Acetylcholine induced endothelium-dependent contractions in SHR but not in WKY, which were inhibited by atropine but not by hexamethonium. These contractions were prevented by cyclooxygenase inhibitors but not by inhibitors of prostacyclin synthetase, thromboxane synthetase, or leukotriene synthetase. Prostaglandins D2, E1, E2, and F2α caused concentration-dependent contractions in both strains, with similar responses at the highest concentration. Endothelium-dependent relaxations to high concentrations of acetylcholine were significantly reduced in SHR compared to WKY, and indomethacin normalized these relaxations in SHR. The findings suggest that acetylcholine activates muscarinic receptors in the aorta of SHR, leading to endothelium-dependent contractions mediated by cyclooxygenase products other than prostacyclin or thromboxane A2. The reduced endothelium-dependent relaxations in SHR are likely due to the simultaneous release of endothelium-derived contracting substances rather than decreased release of relaxing factors.This study investigates the mechanism behind the decreased endothelium-dependent relaxations in the aorta of spontaneously hypertensive rats (SHR) compared to normotensive Wistar-Kyoto rats (WKY). Rings of thoracic aorta from both strains were isolated and suspended for isometric tension recording. Acetylcholine induced endothelium-dependent contractions in SHR but not in WKY, which were inhibited by atropine but not by hexamethonium. These contractions were prevented by cyclooxygenase inhibitors but not by inhibitors of prostacyclin synthetase, thromboxane synthetase, or leukotriene synthetase. Prostaglandins D2, E1, E2, and F2α caused concentration-dependent contractions in both strains, with similar responses at the highest concentration. Endothelium-dependent relaxations to high concentrations of acetylcholine were significantly reduced in SHR compared to WKY, and indomethacin normalized these relaxations in SHR. The findings suggest that acetylcholine activates muscarinic receptors in the aorta of SHR, leading to endothelium-dependent contractions mediated by cyclooxygenase products other than prostacyclin or thromboxane A2. The reduced endothelium-dependent relaxations in SHR are likely due to the simultaneous release of endothelium-derived contracting substances rather than decreased release of relaxing factors.