Introduction Fruits and vegetables contain several types of polyphenols, among them quercetin and its glycosylated products being physiologically important ones. Previous studies have demonstrated their direct vasodilatory capacity on larger arteries. Vasoactive actions on small resistance arteries, however, are still poorly defined. Methods Hearts from male Wistar rats (350-400g) were removed in pentobarbital anaesthesia (50 mg/kg, i.p.), and intramural coronary arterioles were prepared. Segments were cannulated at both ends and immersed in normal Krebs-Ringer solution in a glass-bottomed tissue bath of a pressure angiometer. The bath was thermostated and the solution was bubbled with a mixture of 20% O2, 5% CO2 and 75%N2.Inner and outer diameters were measured videomicroscopically at 50 mmHg intraluminal pressure in Krebs-Ringer solution (spontaneous tone) and with 100nM quercetin added. To test the effect of endogenous vasoactive prostanoids indomethacine (10µM) was applied. L-nitro-methil-arginine (100µM) was added to test a potential endothelial effect. Relaxed diameters were measured in calcium-free solution. Smaller arterioles (inner diameter in Krebs-Ringer solution at 50mmHg 103±9µm, n=7) and greater arterioles (201±11µm, n=9) were separately evaluated. Results In accordance with previous observations, spontaneous tone of the two groups were substantially different (25.6±3.4 % vs. 13.5±1.7 %, p<0.05). Quercetin significantly reduced the tone in both groups (46.7±9.9% and 45.7±9.1%). Indomethacine significantly increased the tone of quercetin treated, relaxed vessels by 44.7±9.9% and 45.3±14.5%. Further, addition of l-nitro-methil-arginine still induced significant relaxation (18.9±8.4% and 27.8±8.2%, p<0.05). Statistical test was one-way ANOVA. Conclusion A physiologically relevant concentration of the flavonoid quercetin significantly decreases the spontaneous tone of intramural coronary arterioles. A substantial part of this vasorelaxation effect is connected to production of vasodilatory endogenous prostanoids while an endothelial NO-mediated mechanism seems to be negligible. These observations suggest the possibility that quercetin appearing in blood plasma from the food induces the operation of a coronary vasodilatory mechanism whose effectiveness is commeasurable with that of the known endothelial actions. (Supported by OTKA 32019, 42670, the Hungarian Hypertension Society, and the Hungarian Kidney Foundation)