Quercetin is a flavonoid found in apples, berries, and onions. Epidemiological studies have found an inverse relationship between quercetin intake and risk of cardiovascular disease and stroke. Quercetin supplementation has been found to reduce severity of hypertension (HTN), improve endothelial and vascular smooth muscle function, and decrease angiotensin converting enzyme (ACE) activity in vivo and in vitro in animal models. Recently, quercetin’s ability to protect against cardiovascular disease in humans has been studied. Evidence suggests that quercetin reduces blood pressure (BP) in hypertensive humans; however, the mechanisms for this phenomenon remain unclear. Furthermore, previous studies have used quercetin supplementation over a period >28 days and it is unclear if quercetin reduces BP acutely or if long-term supplementation is required. The purpose of my research was to determine the effects of a single 1095 mg quercetin aglycone supplement on BP, heart rate (HR), and ACE activity, in normotensive (n = 5) and hypertensive ( n = 12) men. Markers of endothelial function (flow mediated dilation (FMD) and concentrations of endothelin-1 (ET-1) and nitric oxide (NO) metabolites) were also measured in the hypertensive participants. A secondary purpose was to determine bioavailability and the metabolism rate of a single 1095 mg dose of quercetin. This was the first study to examine the effects of a single quercetin dose on cardiovascular and endothelial function in hypertensive individuals. We found10 hours post ingestion of quercetin SBP, DBP, and MAP (mm Hg) significantly (P < 0.05) reduced, without a change in HR or cardiac output, in hypertensive but not normotensive men. Furthermore, the decrease in SBP after quercetin supplementation significantly (P = 0.03) correlated with initial severity of systolic HTN. Reductions in BP occurred without a change in ACE activity, FMD, or concentrations of nitrates + nitrites or ET-1. A time-course analysis in normotensive men revealed increased plasma quercetin concentration within 3 hours of supplementation, maximal plasma concentrations at approximately 10 hours, and return to initial presupplementation concentration within 24 hours. The study’s main finding provides support for the beneficial effects of quercetin on BP reduction in hypertensive men and suggests that quercetin acts after a single dose.