1 The oral bioavailability of quinidine was evaluated in eight patients with moderate to severe congestive heart failure. Each patient was given a 400 mg dose of quinidine gluconate by intravenous infusion and orally in solution. Serial plasma samples and total urine for drug analysis were collected for 24 and 48 h after drug administration, respectively. 2 When compared to control cardiac patients, the rate of quinidine absorption was slower in the heart failure patients. The mean value for the apparent absorption half-life and time to achieve peak plasma quinidine concentration was 38 +/- 18 min and 2.4 +/- 1.5 h respectively. The corresponding values observed in the control subjects were 18 +/- 6 min and 1.0 +/- 0.6 h. 3 The extent of quinidine absorption when evaluated by the AUC and urinary excretion methods was about 72% of the administered dose in the congestive heart failure patients. This value was similar to the extent of quinidine absorption (approximately 73%) observed in the control subjects. 5 When compared with non-heart failure cardiac patients, the results of this study suggest that patients with congestive heart failure may require smaller oral quinidine dosages to achieve therapeutic drug concentrations in the plasma or serum.