The therapeutic effectiveness of aprindine (Amidonal) was evaluated by trend analyses in 25 patients with predominantly severe forms of ventricular extrasystoles. The arrhythmia was stopped or improved in most instances. Severe side-effects for which the drug had to be discontinued occurred in three patients. The drug has a long half-life (about 20-30 hours). As a result, there can be cumulative dose-dependent side-effects, largely of a CNS type, disappearing after the dose has been reduced. Thirteen patients continued to be treated on an ambulatory basis, the oral maintenance dose averaging 100 mg/d, the mean follow-up period 11 1/2 months. There was no evidence of side-effects on liver, electrolytes, renal function or blood.