Since patients vary in their response to furosemide, a pharmacokinetic study of this drug was undertaken in 7 patients with severe congestive heart failure. The extent of oral availability, determined by the ratio of areas under the oral to intravenous plasma concentration-time curve, averaged 61%, but varied widely from patient to patient, from 34 to 80%. Absorption of the drug was often delayed, with a lag time before appearance in the plasma averaging 28 +/- (SD) 15 min. The volume of distribution of furosemide (averaging 114 ml/kg) was not diminished in the presence of congestive failure; likewise, drug clearance and elimination half-lives are similar to values reported in normal subjects. Urinary flow rates 1--2 h following dosing were significantly less after oral than intravenous dosing, but flow rates over 4, 8 or 24 h after dosing were similar for both routes of administration.