The absolute bioavailability of quinidine was studied in 11 hospitalized patients. A 400-mg dose of quinidine gluconate was administered to each patient by intravenous infusion and as an oral solution. Drug treatments were separated by a 72-hr period. In 8 patients, peak plasma quinidine concentrations were reached in 65 min after the oral dose; in the remaining 3 subjects, peak concentrations were reached later. From the ratio of the total area under the plasma concentration-time curves (AUCoral/AUCir), the absolute bioavailability of quinidine ranged from 44% to 89% (mean, 72). In 8 patients, the ratio of the total amount of quinidine excreted in the urine in 48 hr (AUinfinity oral/AUinfinity ir) indicated that the extent of quinidine bioavailability varied form 47% to 96% (mean, 73). The predicted bioavailability of quindine due to first-pass effects was 76+/-11%. It is concluded that absorption after the oral solution was rapid and that the reduction of quinidine bioavailability was due to first-pass hepatic drug removal.