The pharmacokinetics of terazosin have been assessed in human volunteers, hypertensive patients, a limited number of elderly volunteers, and a small number of patients with congestive heart failure. Terazosin was administered intravenously and orally in doses ranging up to 7.5 mg. Following intravenous administration, the disposition of terazosin is characteristic of a two-compartment, open model that is linear and independent of dose. Orally administered terazosin is rapidly, consistently, and almost completely absorbed into the bloodstream. Peak plasma drug levels occur within one to two hours after ingestion. Approximately 90 to 94 percent of the drug is bound to plasma proteins, with the volume of distribution estimated to be 25 to 30 liters. Terazosin undergoes extensive hepatic metabolism, and the major route of elimination is via the biliary tract. Small amounts of terazosin are excreted in the urine. Plasma and renal clearances are 80 and 10 ml per minute, respectively. The mean beta-phase half-life is approximately 12 hours. The pharmacokinetics of terazosin were not influenced by age, congestive heart failure, or hypertension (other than plasma clearance). In contrast to prazosin, terazosin is completely and consistently bioavailable and has a half-life that is three to four times longer than that of prazosin. The prolonged half-life of terazosin allows once-daily dosing, which may facilitate patient compliance with drug therapy for hypertension.