Hydralazine is known to improve cardiac function in patients with congestive heart failure (CHF), but its effects on renal function in CHF are less clear. Sodium retention is known to occur with long term use of hydralazine to treat hypertension; if this occurs in patients with CHF it could be deleterious. Therefore, in a metabolic unit we studied renal effects of hydralazine in patients with stable class III or IV CHF. In a single-blind study, the patients were given placebo twice daily for 3 days (period P-1), 100 mg of oral hydralazine twice daily for 3 days (period P-H), and placebo for 3 more days (period P-2). The average 24-hour creatinine clearance was 69.7 ± 7.7 ml/min (mean ± SEM) in P-1, increased to 76.3 ± 9.0 ml/min with hydralazine (p<0.01) and fell again when hydralazine was stopped (P-2) to 68.5±7.8 ml/min (p<0.02). Though the slight improvement in sodium excretion was not statistically significant (60.2±12.1 mEq in P-1, 64.5±12.4 mEq in P-H, 52.3±7.7 mEq in P-2), serum osmolality decreased from 288±1.8 mosM in P-1 to 283±1.9 mosM in P-H (p<0.02) and rose to 286±1.9 mosM in P-2 (NS). During the three periods, serum sodium, chloride, potassium, carbon dioxide, blood urea nitrogen, creatinine ang glucose were unchanged, as were weight and urine volume. Systolic blood pressure was 109.6±3.6 mm Hg in P-1, 110.1±3.9 mm Hg in P-H (NS), and 114.2±5.0 mm Hg in P-2 (p<0.05. Diastolic blood pressure, heart rate and respirations were unchanged. Thus, we found no evidence of sodium or water retention during hydralazine administration in patients with CHF, and renal function was actually improved, as evidenced by the inceased creatinine clearance.