In congestive heart failure, cardiac output is low, blood pressure is high, and the body becomes congested with fluid. In normal people, when there is high blood pressure, the heart muscle cells secrete a hormone that excretes sodium and water in the urine, reducing blood pressure. The action of this hormone is called the natriuretic response. The purpose of this study is to determine if nesiritide can improve an impaired natriuretic response in subjects with asymptomatic systolic heart failure or asymptomatic diastolic heart failure.