Sildenafil (Viagra) is known to reduce pulmonary hypertension. Heart failure patients also have pulmonary hypertension and several recent reports have shown that sildenafil leads to an improvement in their exercise capacity. In these studies sildenafil caused a reduction in the pulmonary and systemic vascular resistances, improved pulmonary gas diffusion and perhaps increased cardiac output. It is uncertain if left ventricular filling pressures are reduced and whether there is improvement in left ventricular relaxation. The investigators hypothesize that in heart failure patients the improvement in exercise capacity associated with sildenafil is related to a significant reduction in left ventricular filling pressures. The investigators propose to study 20 patients with stable but moderately symptomatic heart failure. The study design is a randomized cross-over trial of the administration of a single dose of sildenafil 50 mg or a matching placebo. Exercise capacity will be determined before and after the oral administration of sildenafil 50 mg or placebo. Left ventricular filling pressures will be assessed by Doppler echocardiography and the serum level of B-type natriuretic peptide (BNP is known to increase with higher left ventricular filling pressures). After an initial echocardiogram and performing a 6 minute walk test, the patient will then be given either sildenafil or a matching placebo in a randomized double-blind fashion. One hr later a blood sample for serum BNP, the echocardiogram and the 6 minute walk test will be repeated.