Dilation of the pupil was obtained with tropicamide (Mydriatikum Roche; Hoffmann-La Roche, Vienna, Austria). After a resting period of at least 20 minutes baseline measurements of ocular and systemic hemodynamics were performed. In addition, a 60-second flicker period with an 8-Hz flicker stimulus was scheduled and retinal vessel diameters were measured continuously. Thereafter, subjects received a single dose of 100 mg sildenafil or placebo. Ocular and systemic hemodynamic parameters were measured every 20 minutes over 3 hours and finally after 6 hours. Intraocular pressure (IOP) was measured in the fellow eye with applanation tonometry at baseline, every 60 minutes over 3 hours and after 6 hours. Sixty minutes after administration of sildenafil, vessel diameter were again assessed during flicker stimulation with 8 Hz over 60 seconds. Blood pressure was measured in 10-minute intervals during the entire study period. Pulse rate was monitored continuously. At each measurement cycle, the methods were applied in a predetermined order (RVA, blood pressure and pulse rate, LDV, IOP, if applicable). Measurements with the RVA and LDV were taken over 60 seconds each. Each method was performed by the same experienced observer throughout the study. The studies were performed at the Department of Clinical Pharmacology, Allgemeines Krankenhaus, Vienna.