The acute effects of nitroprusside infusion and intravenous hydralazine on renal hemodynamics and function were evaluated in nine male patients with severe, low cardiac output, congestive heart failure (CHF). Both drugs resulted in marked systemic hemodynamic improvement. Nitroprusside had a more profound effect on pulmonary artery pressure, while hydralazine produced a greater elevation in cardiac output. Significant decreases in both systemic and pulmonary vascular resistance and pulmonary capillary wedge pressure with increases in stroke volume and stroke work index were noted with both drugs. Total renal resistance decreased and renal blood flow (RBF) significantly increased with both drugs, while the distribution of cardiac output to the kidney remained depressed. Glomerular filtration rate (GFR) did not change significantly with either drug, although increases in GFR were seen in selected patients in whom RBF increased by more than 10% from control. The fraction of plasma filtered decreased toward normal with both drugs and excretion of total cations was significantly increased. These changes all represent improvements in systemic and renal hemodynamic abnormalities occurring in patients with CHF; their maintenance during long-term therapy would facilitate patient management.