We have studied the effects of quinidine on ECG intervals and on the pharmacokinetics of flecainide and its two metabolites in 6 healthy men in an open randomized crossover study. Flecainide acetate (150 mg) was given as a constant rate i.v. infusion over 30 min. Quinidine (50 mg orally), given the previous evening, did not change the volume of distribution of flecainide (7.9 vs 7.4 l.kg-1), but significantly increased its half-life (8.8 vs 10.7 h). This was attributable to a reduction in total clearance (10.6 vs 8.1 ml.min-1 x kg-1), most of it being accounted for by a reduction in non-renal clearance (7.2 vs 5.2 ml.min-1 x kg-1). The excretion of the metabolites of flecainide over 48 h was significantly reduced. These findings suggest that quinidine inhibits the first step of flecainide metabolism, although it may also reduce its renal clearance, but to a lesser extent (3.5 vs 2.9 ml.min-1 x kg-1). The effects of flecainide on ECG intervals were not altered by quinidine. Thus, quinidine tends to shift extensive metabolizer status for flecainide towards poor metabolizer status and may also alter its renal excretion.