8-Chloro-6-[(1-isopropyl-3-imidazolin-2-yl)-methyl]-1,6-benzoperhydrothiazocin-hydrochloride (dazolicin, ucb B 192) is a new antiarrhythmic drug with direct membrane action which was applied both orally and parenterally. The immediate antiarrhythmic effect of a single i.v. injection of 150 mg of dazolicin on the average was investigated in 28 patients with various types of arrhythmia. After i.v. injection the drug proved to have very strong antiarrhythmic potency and rather a low incidence of side effects. Ectopic beats and paroxysmal tachycardias of both ventricular and supraventricular origin were successfully treated with dazolicin. The antiarrhythmic drug significantly increased the duration of both the QRS- and QT-interval after correction for frequency but it had no detectable effects on the atrioventricular conduction time. After i.v. administration the antiarrhythmic effects of the drug lasted for several hours. The elimination half-life of dazolicin was 7 h. Oral treatment with dazolicin was attempted in 10 patients suffering from stable extrasystolic arrhythmia with daily doses ranging from 3 x 25 mg to 3 x 50 mg. In only 4 patients ectopic beats could sufficiently be eliminated. According to the low dosage the maximum serum concentrations after oral application were significantly lower than after i.v. injection. In two patients serious side effects were observed, such as paroxysmal ventricular fibrillation and an increase in frequency and polymorphism of ventricular ectopic beats. In both instances the patients were suffering from congestive heart failure and they had TU abnormalities in the ECG.