The effects of carteolol, propranolol and alprenolol were studied by using spontaneously contracting atria, atrial muscle and papillary muscle prepared from rat hearts. Propranolol, alprenolol and carteolol caused decreases in atrial rate and in contractile tension of electrically driven cardiac muscles in concentrations of 10(-6) to 10(-5) g/ml, 10(-6) to 10(-5) g/ml and 10(-4) g/ml, respectively. These compounds, also, effectively prevented electrically-induced atrial arrhythmias in these concentrations. Contractile tension of the cardiac muscles was increased by carteolol in concentrations of 10(-7) to 10(-6) g/ml. The refractory period of the cardiac muscles was prolonged in the presence of alprenolol (10(-7) to 10(-6) g/ml) or carteolol (10(-6) to 10(-5) g/ml). The refractory period of the papillary muscle was increased by 10(-7) to 10(-6) g/ml of propranolol. The maximum driven frequency (MDF) of the atrial muscle was reduced by 5X10(-8) to 10(-7) g/ml of carteolol or by 5X10(-8) g/ml of alprenolol, and MDF of the papillary muscle by 5X10(-7) g/ml of propranolol. In extremely high concentrations, these compounds significantly reduced MDF of the cardiac muscle. These results suggest that the antiarrhythmic action of alprenolol and carteolol is more predominant in atria than in ventricles, whereas the reverse is true for propranolol.