24-h ECG recordings were used to assess the efficacy of prajmalium bitartrate (PB) in reducing the incidence and the severity of premature ventricular complexes (PVCs), and to compare its antiarrhythmic action with that of Disopyramide. 13 patients with frequent PVCs were distributed randomly into 2 groups. The first group of 7 patients received PB 80 mg/day for 4 days as their first treatment, and disopyramide 400 mg/day for a further 4 days as the second therapy. The succession of the drugs was reversed in the other group of 6 patients. Analysis of the Holter recordings showed that PB and disopyramide reduced PVC frequency to a similar extent as compared to the corresponding wash-out period, viz. by 56.7% (p less than 0.05) and 62.1% (p less than 0.01), respectively. Thus, PB appears to be an effective antiarrhythmic drug and comparable to disopyramide. It may be used to prevent premature ventricular complexes and runs of ventricular tachycardia.