20 patients with chronic premature ventricular contractions (P.V.Cs) underwent several Holter ECG monitorings to assess clinical effectiveness of three antiarrhythmic drugs: Prajmalium Bitartrate (P.B.) 80 mg/daily, Disopyramide (D.) 600 mg/daily and Procainamide (P.) 2400 mg/daily, to assess the most effective antiarrhythmic medication in every patient. Clinical effectiveness was considered as 80% reduction of P.V.Cs or 50% reduction with suppression of all complex ventricular ectopy (repetitive, polymorph, bigeminy). These results were observed respectively, for 80% reduction, in 5/20 patients for P.B., in 9/20 for D., and in 3/19 for P; and for 50% reduction in 11/20 for P.B., in 18/20 for D., and in 9/19 for P. Comparison in the same patient, using Holter ECG monitoring with a computer assisted analysis, of the effects of different antiarrhythmic medications, is a rational procedure to assess clinical efficacy of new antiarrhythmic drugs and to choose the most effective in each case.