Cibenzoline succinate [4,5-dihydro-2-(2,2-diphenylcyclopropyl)imidazole butanedioate (1:l) salt] is a new antiarrhythmic drug. It is not chemically related to other known antiarrhythmic agents ( Fig. 1) and is the first imidazoline derivative known to have antiarrhythmic activity. The molecular weight of cibenzoline succinate is 380, and its empiric formula is CI8Hl8N2-C4H6O4. Cibenzoline has also been referred to in the literature with the trade names Cipralan@, RitmalanB, and Pracizolinem and the industrial codes UP 339.01 and RO 22-796/001.Cibenzoline has the important advantage of being effective by either oral or intravenous administration. The efficacy of cibenzoline against ventricular arrhythmias has been established in both experimental (11,19,31) and clinical (1,3,5,25,29,40) studies. The drug has been reported to have few side effects during chronic oral administration in clinical studies (5,18,40,53,54). Cibenzoline has been placed into categories 1, 3, and 4 of the Vaughan Williams classification system for antiarrhythmic agents on the basis of its effects on isolated tissues from the rabbit heart (37).