To test the effect of trimethoprim (an antibiotic commonly administered with sulfamethoxazole) on the disposition of the antiarrhythmic procainamide hydrochloride and its active metabolite N-acetylprocainamide, 10 healthy men received 1 g of procainamide hydrochloride orally on two occasions, coadministered with placebo or trimethoprim (100 mg twice a day for 2 days before and then 200 mg with the procainamide dose). Trimethoprim decreased the mean (+/- SD) renal clearance by 45% after the dose of procainamide was administered (487 +/- 129 vs 267 +/- 123 mL/min) and that of N-acetylprocainamide by 26% (275 +/- 78 vs 192 +/- 82 mL/min) compared with placebo. The mean area under plasma concentration--time curve 0 to 12 hours after dosing increased 39% for procainamide (19.8 +/- 4.8 vs 27.6 +/- 7.2 mg.h/L) and 27% for N-acetylprocainamide (9.1 +/- 2.1 vs 11.4 +/- 2.8 mg.h/L). The corrected QT electrocardiographic interval at 2 hours after the procainamide dose was 0.40 +/- 0.02 second with placebo and 0.43 +/- 0.03 second with trimethoprim. Trimethoprim may increase procainamide and N-acetylprocainamide plasma concentrations, resulting in increased pharmacodynamic response apparently caused by the competition for renal tubular cationic secretion.