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 less than 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 infinity 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 less than 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.(ABSTRACT TRUNCATED AT 250 WORDS)