Isoptin and mesotrin (a new hypothetical calcium antagonist) were administered to 30 patients with frequent and persistent premature beats (22 patients with premature ventricular beats and 8 patients with premature supraventricular beats). The antiarrhythmic activity of mesotrin was nearly twice as high as that of isoptin, the antiarrhythmic effect being achieved in 63 and 33% of patients, respectively, which was especially distinct in ventricular rhythm disturbances (64 and 27%, respectively). The effective dose of mesotrin was approximately 2.5 mg/kg. Isoptin produced the greatest effect on atrio-ventricular conduction, while mesotrin to a great extent influenced atrial and ventricular conduction. Certain side effects of mesotrin were observed, consisting in slight euphoria, ventigo and heavyheadedness subduing spontaneously 10-15 minutes after administering the drug. Care should be taken when administering the drug to patients with sick sinus syndrome because of the possibility of its arrest.