Vardenafil (Levitra) and Sildenafil (Viagra) are drugs that are marketed for use in patients with erectile dysfunction. The purpose of this study is to find out if there are any changes in the eye after taking at least 15 doses of Vardenafil and after two doses of Sildenafil. This is a double-blind study, which means that neither you nor the study doctor will know which treatment you are receiving. If you qualify for the study, you will receive:- Vardenafil 20 mg twice per week (Monday and Thursday) for a maximum of 8 weeks. There will be two times when this occurs during the study.- Sildenafil 200 mg/day for two days. There will be two times when this occurs during the study. - Placebo (a pill that looks like vardenafil or sildenafil but has no active ingredient).

Inclusion Criteria:- Healthy adult male subjects between 18 and 55 years of age, inclusive, at screening- Body mass index (BMI) 19 to 34 kg/m2- Negative for HIV, Hepatitis B and Hepatitis C at screening Exclusion Criteria:- Any unstable medical, psychiatric, or substance abuse disorder- History of moderate or severe hepatic impairment- Creatinine clearance- A resting systolic blood pressure of < 100 mm Hg or > 170 mm Hg or a resting diastolic blood pressure > 110 mm Hg- Blindness, color blindness or vision in either eye judged as abnormal by the investigator- History of trauma or surgery to either eye, of retinitis pigmentosa, of diabetes or current diabetes, of epilepsy or of seizure disorders- Symptomatic hypotension within 6 months of start of study- History of sickle cell anemia or sickle cell trait or bleeding disorder- Significant active peptic ulceration or other gastrointestinal disorder such as colitis or Crohn's Disease- Use of alpha blockers, nitrates, ritonavir, indinavir, ketoconazole, itraconazole, or erythromycin, ciprofloxacin, tadalafil, isotretinoin, ibuprofen, unoprostone isopropyl, Lily-of-the-Valley, Strophanthus, ranitidine and sildenafil