A comparative bioavailability study was performed using four commercially available, chemically equivalent brands of quinidine sulfate tablets. Two 200-mg tablets were administered to 11 different subjects following a completely randomized crossover design. Serum levels, urinary excretion data, and derived pharmacokinetic parameters were compared statistically. There were no statistical differences in the extent of quinidine absorption from the four brands of tablets as evidenced by the cumulative urinary excretion values and the areas under the serum level-time curves. Significant differences in the mean serum levels at 0.5 and 1 hr and differences in the peak times and absorption rate constants indicate that there was a difference in the absorption rate between Treatments A and D and C and D. A significant difference in the peak times also was noted for Treatments B and C. When mean disintegration times for the four tablet formulations were compared with their values for ka, tmax and mean serum levels at 0.5 and 1 hr, rank-order correlations were observed. A considerable degree of variability in quinidine elimination was noted, with half-life values ranging from 2.71 to 8.12 hr (mean half-life of 5.36 hr).