The bioavailability of three quinidine formulations was estimated during a dosing interval at steady state following their administration in 12 selected patients in accordance with a Latin-square 3 x 3. Each subject received the three dosage forms as two tablets every 12 hr for 6 days. Blood and urine samples were taken on the 7th day during the regular 12-hr dosing interval. Unchanged quinidine was determined by a reported spectrofluorometric procedure. The total fluorescence of plasma quinidine and metabolites also was monitored. The data obtained indicate that one dosage form gave a high peak level followed by a fall in the concentration. The two other forms presented a relatively lower peak followed by a plateau and then a decline. The differences between the dose-corrected values of Cp,max were statistically significant. Secondary effects were observed particularly with one dosage form and could be related to the high Cp,max value and/or the high percentage of quinidine liberated rapidly in the GI tract. Blood and urinary data indicated an equivalent degree of absorption. The dissolution behavior of the formulations and their absorption data suggest that there is a correlation between the quantity dissolved at 30 min and Cp,max.