This research study, funded by Scios, Inc., and conducted by the Cardiology Division of the Department of Medicine at the University of Rochester School of Medicine and Dentistry investigates the potential benefit of nesiritide (brand name is Natrecor(TM))in improving exercise capacity in patients with heart failure (HF). Previous studies have shown beneficial hemodynamic and neurohumoral effects of nesiritide infusion in the therapy of decompensated heart failure. Other studies have demonstrated increases in endogenous BNP levels (normalized for exercise workload) in HF patients during and after exercise. However, trials involving the measurement of exercise capacity in this population following BNP administration are lacking. This is an experimental prospective, non-blinded, pilot study with a sample size of 20 patients, all serving as their own controls. This study involves off-label use of the drug nesiritide (indicated for IV treatment of NYHA Class II and III patients, and this study is enrolling Class II and III patients) and has received FDA approval, and an IND. Subjects are recruited from the population of referrals to the Strong Memorial Hospital Heart Failure and Transplantation clinical service, who meet all the inclusion criteria and none of the exclusion criteria may participate in the study. Enrolled study subjects perform an exercise VO2 max test, receive a 24 hour infusion of nesiritide and perform a second exercise VO2 max test.