Hypertension (persistently elevated blood pressure) is a major risk factor for heart disease and stroke. Hypertensive individuals show a reduced exercise capacity, which is present from a very early stage and contributes to their increased cardiovascular risk. Sildenafil belongs to a class of drugs called phosphodiesterase type 5 (PDE5) inhibitors, and it works by enhancing the effects of nitric oxide, a substance that dilates blood vessels and increases blood flow. We hypothesize that sildenafil, because of its effect on nitric oxide and blood flow, will improve exercise capacity in hypertensive patients. Therefore, the main aim of the study is to investigate the effects of PDE5 inhibition on exercise capacity and vascular function in hypertension, and to compare these effects in hypertensive patients and healthy controls.