This is a randomized, double-blind, placebo controlled study of low dose nesiritide to prevent renal insufficiency after cardiopulmonary bypass surgery. Patients included are those with greater than mild renal insufficiency preoperatively as defined by having a creatinine clearance of less than 50 mL/minute determined by the Cockroft-Gault formula. The infusion of nesiritide (BNP) 0.005 mcg/Kg/minute or placebo begins 1 hour before surgery and runs for 24 hours started after the induction of anesthesia prior to cardiopulmonary bypass.