We determined the efficacy, pharmacokinetics, and plasma concentration-response relationships of propafenone, a promising new antiarrhythmic drug. Thirteen patients with frequent and complex ventricular premature beats were studied after receiving four increasing doses, during drug washout and during a randomized double-blind placebo-controlled trial, to evaluate the optimal dose in each patient. A nonlinear relationship was found between propafenone dose and steady-state mean concentration with a 10-fold increase in drug concentration as dose increased threefold from 300 to 900 mg/day. There was great intersubject variability in elimination half-life (mean 6 hr, range 2.4 to 11.8), steady-state mean concentration on 900 mg/day of propafenone (mean 1008 ng/ml, range 482 to 1812), and "therapeutic" plasma concentration (mean 588 ng/ml, range 64 to 1044). The interaction of these three parameters in individual patients determined the duration of the antiarrhythmic action of propafenone during washout (mean 11.5 hr, range 4 to 22). There was a greater than 90% reduction of ventricular premature beats in 10 subjects during dose ranging and in seven during double-blind crossover. Side effects requiring discontinuation of the drug occurred in three patients and included apparent worsening of arrhythmias in two. We conclude that propafenone effectively suppresses ventricular arrhythmias and that nonlinear drug accumulation and intersubject variability in elimination of half-life, steady-state mean plasma concentration, and therapeutic concentration indicate a need for individual therapy.