3',4'-dihydroxyflavonol (DiOHF) is a potent antioxidant that reduces infarct size following myocardial ischaemia-reperfusion. Since oxidative stress induced by myocardial ischaemia-reperfusion impairs endothelium-dependent vasodilatation, we investigated whether DiOHF preserved coronary endothelial function following ischemia-reperfusion. One week after surgery conscious, instrumented sheep were subjected to 1h of myocardial ischaemia followed by 7 days reperfusion. Immediately before reperfusion, sheep were injected with DiOHF (2mg/kg iv, n=4) or vehicle (dimethyl sulphoxide, n=4). Coronary vascular responses to the endothelium-dependent vasodilator acetylcholine (ACh, 0.05-10.0 microg/kg/min iv), sodium nitroprusside and phenylephrine were determined. After ischaemia-reperfusion, dP/dt(max) decreased from 1511+/-93 to 1094+/-53 mmHg/s, P<0.05) at 24h in the vehicle group, but by 7 days had returned towards baseline (1347+/-91 mmHg/s). DiOHF prevented the fall in dP/dt(max). Coronary conductance (CC) was increased (+34+/-4%) by 10 microg/kg ACh given before ischaemia, but this vasodilatation was significantly reduced after 24h and 7 days of reperfusion (+7+/-2%, +15+/-2%, respectively, both P<0.05). DiOHF partially preserved the coronary vasodilator response to ACh after 24h reperfusion (basal 37+/-7%, 24h 18+/-5%), and after 7 days reperfusion the response had recovered (31+/-7%). DiOHF significantly decreased infarct size, expressed as a percentage of area-at-risk, by 40% after 7 days reperfusion (vehicle 80+/-7%, DiOHF 46+/-11%, P<0.05). A single administration of DiOHF, during ischaemia and just prior to reperfusion, reduced infarct size, preserved ventricular contractility and caused a sustained protection against coronary endothelial dysfunction, with all these beneficial actions being preserved for 7 days reperfusion.