Electrical effects of the new antiarrhythmic compound Bonnecor (AWD 19-166, GS 015) were studied under conditions known to increase the intracellular Ca2+ activity (extracellular K+-deprivation, application of glycosides). In K+ free solutions right ventricular papillary muscles showed an extreme acceleration of the initial repolarization indicating an intracellular Ca overload. AWD 19-166 could not prevent the initial shortening of action potentials and seems to be less effective in K+ free than in K+ containing solutions. In partially depolarized (22 mmol/lK+) right ventricular papillary muscles oscillatory afterdepolarizations were not suppressed by AWD 19-166. However, the compound prevented the generation of irregular action potentials triggered by the afterdepolarizations.