The absorption characteristics of 3 sustained release quinidine formulations were assessed in 12 healthy male volunteers in a randomised 3-way crossover trial. Each formulation (‘Quinidex’ 300mg, ‘Biquin Durules’ 250mg and ‘Quinaglute Dura-Tabs’ 324mg) was administered as a single tablet every 12 hours for 5 days. Peak quinidine serum concentrations of 2.7 ± 0.8 mg/L occurred 2.5 ± 1.1 hour after ‘Quinaglute’ administration, significantly higher (p < 0.01) than concentrations of 1.6 ± 0.4 mg/L achieved 4.2 ± 1.1 hours following ‘Biquin’ dosing and 1.7 ± 0.6 mg/L attained 3.9 ± 2.7 hours after ‘Quinidex’ ingestion. The extent of absorption based on AUC∞and normalised for the anhydrous quinidine content was similar for the 3 products. Following multiple dosing, the mean steady-state trough concentration of quinidine was 2.06 ± 0.56 mg/L for ‘Quinidex’, significantly greater (p < 0.05) than that of ‘Biquin’ (1.18 ± 0.67 mg/L) or ‘Quinaglute’ (1.58 ± 0.58 mg/L).The rate of absorption was found to be much slower for ‘Quinidex’ than for the other 2 sustained release quinidine formulations. Comparison of the residual sums of squares from simple linear regression of Wagner-Nelson plots did not demonstrate a preference for a zero- or first-order absorption model. Nevertheless, the absorption of ‘Quinidex’ was twice as prolonged as that of ‘Biquin’ and ‘Quinaglute’ regardless of model; first-order absorption half-lives were 2.83 ± 1.02 hours, 1.25 ± 0.6 hours and 1.43 ± 0.88 hours, respectively. The data also suggest that ‘Quinidex’ absorption may continue beyond 12 hours in some subjects.These results indicate that the rate of quinidine absorption from various sustained release formulations may be markedly different. Since these formulations also differ in their anhydrous quinidine content, prescribers should not view these products as equivalent or interchangeable.