 This study will evaluate the efficacy and safety of administering an angiotensin converting enzyme inhibitor ACE-I enalapril to infants with a functional single ventricle. The study will also compare the effect of ACE-I therapy to placebo on somatic growth and compare the effect of ACE-I therapy to placebo on signs and symptoms of heart failure, neurodevelopmental and functional status, ventricular geometry, function, and atrioventricular AV valve regurgitation. In addition, the study will determine the relationship between genetic polymorphisms linked to ventricular hypertrophy enlarged heart and the response to ACE-I therapy and compare the incidence of adverse events in subjects treated with ACE-I with those in subjects treated with placebo. 