The diuretic effect of furosemide was studied in 18 patients with congestive heart failure. Subjects were divided into two groups, group I consisting of eight patients with moderate and group II of 10 patients with advanced congestive heart failure. Six hours after bolus injection of furosemide (40 mg), mean urinary sodium was 120.5 +/- 36.7 mEq in group I and 68.2 +/- 25.8 mEq in group II (p less than 0.01), mean urine volume was 1,100 +/- 281 and 764 +/- 257 ml (p less than 0.05), mean urinary furosemide excretion was 28.08 +/- 2.60 and 24.00 +/- 0.74 mg (p less than 0.05), and mean furosemide renal clearance was 73.4 +/- 16.6 and 42.3 +/- 11.5 ml/min (p less than 0.001). Diuretic effect and furosemide renal clearance, as well as urinary furosemide excretion, correlated positively. The diuretic effect of furosemide with and without hydralazine (0.2 mg/kg) was compared in eight patients in group II. Urinary sodium excretion 6 hr after furosemide rose from 77.2 +/- 31.0 to 122.8 +/- 42.5 mEq after furosemide with hydralazine (p less than 0.01). Urine volume rose from 854 +/- 278 to 1,279 +/- 359 ml (p less than 0.001), urinary furosemide excretion rose from 23.64 +/- 2.03 to 26.94 +/- 2.30 mg (p less than 0.01), and furosemide renal clearance rose from 46.3 +/- 12.2 to 62.5 +/- 18.6 ml/min (p less than 0.01).