This pilot study is aimed at assessing the short-term effects of Tadalafil on the hemodynamic response to exercise and exercise capacity in patients with Fontan circulation. Data regarding effect size and drug tolerability will be used in the design of a randomized multicenter trial. The long-term goal of this investigation is to systematically evaluate the effect of tadalafil therapy on exercise performance, quality of life, and delay of functional deterioration in patients with single ventricle physiology.

Inclusion Criteria:

• Patients who have undergone the Fontan procedure (regardless of type of Fontan connection or presence or absence of a fenestration)
• Age 8.0 to 35.0 years
• Stable clinical condition over the last 3 months (i.e. No change in medication, treatments, or development of new symptoms)
• Ability to perform exercise testing
• Consent and assent (as appropriate to participate in the study after receiving information concerning procedures, risks, and possible clinical benefits) o Patients will be enrolled without regard to gender, race, and ethnicity. All patients meeting study eligibility will be approached for consent.

Exclusion Criteria:

• Severe heart failure (New York Heart Association functional class III or IV)
• Presence of liver or renal dysfunction based on the latest lab test results
• Presence of hearing or visual deficit
• Transcutaneous arterial blood oxygen saturation (SaO2) <80% at rest
• History of echocardiographic, MRI, or angiographic evidence of Fontan pathway obstruction
• History of exercise-induced life-threatening arrhythmias or unstable rhythm(i.e. atrial flutter)
• Known or suspected pregnancy. Females in the reproductive age group will be screened for pregnancy using serum beta hCG prior to administering study drug.
• Patients on open label sildenafil or tadalafil