A Randomized, Double-Blinded, Placebo-Controlled, Two-Way Crossover Study to Investigate the Safety and Toleration of Single Dose Sildenafil in Subjects Receiving Chronic UK-369,0031

Inclusion Criteria: * Evidence of a personally signed and dated informed consent document indicating that the subject (or a legally acceptable representative) has been informed of all pertinent aspects of the study. * Healthy male subjects between the ages of 40 and 55 years, inclusive * Body Mass Index (BMI) of approximately 18 to 30 kg/m2; and a total body weight \>50 kg (110 lbs). Exclusion Criteria: * Evidence or history of clinically significant haematological, renal, endocrine, pulmonary, gastrointestinal, cardiovascular, hepatic, psychiatric, neurologic, or allergic disease (including drug allergies, but excluding untreated, asymptomatic, seasonal allergies at time of dosing). * Loss of vision in one eye due to non-arteritic ischemic optic neuropathy (NAION) regardless of whether or not this event was temporally associated with the use of a PDE5 inhibitor.