The present experiment was performed to identify endothelium-derived contracting factor produced by acetylcholine stimulation in the aorta of spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto (WKY) rats. The rings of the thoracic aorta were obtained from age-matched SHR and WKY rats, and changes in isometric tension were recorded. The relaxant responses to acetylcholine in the aortic rings from SHR were significantly weaker than those from WKY rats. The relaxant responses to acetylcholine were significantly enhanced by pretreatment with a cyclooxygenase inhibitor (indomethacin) or thromboxane A2/ prostaglandin H2receptor antagonist (ONO-3708) in aortic rings from both SHR and WKY rats. A thromboxane A2 synthetase inhibitor (OKY-046) did not affect the acetylcholineinduced relaxation in the aortic rings from SHR or WKY rats. In the organ bath solution, after acetylcholine stimulation, prostaglandin E2 and 6-keto-prostaglandin Fla concentrations increased but not prostaglandin F2a and thromboxane B2concentrations. Exogenous prostaglandin H2, a stable analogue of thromboxane A2, and prostaglandin F2α induced contractions of the SHR rings at a lower concentration than prostaglandin E2, prostaglandin D2, and prostaglandin I2. These contractile responses to various prostaglandins were markedly inhibited by pretreatment with ONO-3708. A prostacyclin synthetase inhibitor did not affect the relaxant responses to acetylcholine in the SHR rings. These results show that endotheliumderived contracting factor is produced and released by acetylcholine stimulation not only in the aorta of SHR but also in those of WKY rats and suggest that prostaglandin H2, a precursor of the released prostaglandins, is a strong candidate for endothelium-derived contracting factor produced by acetylcholine stimulation.