We studied the efficacy of propafenone in 16 patients (pts) with exercise-induced ventricular arrhythmias (VA). Propafenone was given in doses of 450 mg (13 pts) to 900 mg (3 pts) per day and was compared with placebo. An arrhythmic index was calculated by dividing the frequency and severity of VA by the duration of exercise test (in min). Propafenone reduced that VA index from 17.05 +/- 16.32 to 3.75 +/- 7.47 (P less than 0.005). In 8 pts the VA were completely abolished, and in 8/10 grade greater than or equal to 3 VA, while no significant change was seen with placebo. In 8 pts in whom exercise had been prematurely terminated because of significant VA, a longer duration was achieved with propafenone (from 4.19 +/- 2.74 to 9.39 +/- 1.63, P less than 0.02). In those with the same pre-planned duration, a smaller peak heart rate was seen with propafenone. Resting P-R was significantly prolonged.