The acute intravenous effects of therapeutic doses of procainamide, lignocaine, metoprolol, digoxin and atropine on the monophasic action potentials (MAP) and effective refractory periods of the right ventricle (VERP) were studied in 48 healthy volunteers. Procainamide prolonged the VERP in the apex region. Lignocaine shortened the MAP duration at 90% repolarisation. Metoprolol did not affect any of the measured variables in spite of a significant decrease in heart rate. Digoxin produced a significant increase in the VERP at the outflow tract, but not in the apex region and the MAP variables did not change. Following atropine, the VERP at both recording sites decreased but the MAP signal was unaffected. In summary, the effects of procainamide, lignocaine, metoprolol and digoxin were in good agreement with previous studies in normal ventricular muscle cells in vitro. In addition, the findings following atropine, digoxin and procainamide are indicative of a parasympathetic innervation of the endocardial surface of the right ventricle.