In 11 patients with stable premature ventricular beats, the kinetics of single (150 and 300 mg) and multiple (150 mg t.i.d. and 300 mg t.i.d.) oral doses of propafenone were studied with reference to arrhythmia suppression. During the acute phase detectable plasma levels of the drug were achieved only with the higher dose. In 8 out of 10 patients the antiarrhythmic effect was obtained with the 300 mg dose, which was found to predict responsiveness at steady-state. During the chronic phase, antiarrhythmic efficacy was obtained with the lower dose regimen (150 mg t.i.d.) in half of those patients. A wide range of effective plasma levels was observed. The previously suggested therapeutic range (0.5-2.0 micrograms/ml) was not adequate in predicting either antiarrhythmic activity or adverse effects. The results show the role of propafenone metabolites in determining total antiarrhythmic action.