The effect of acebutolol a beta-adrenergic receptor blocking agent was tested on the dog heart in situ. The drug decreased heart rate, and caused a reduction in the conduction velocity and a significant increase in the functional refractory period of the atrioventricular node. The functional and the effective refractory period of the right atrium was increased by acebutolol. During sinus rhythm, the drug did not affect conduction velocity in the rest of the conduction pathway. During atrial pacing, however, the intra-atrial and His-Purkinje conduction times were slightly increased. The plasma concentrations of acebutolol were in the range between 0.09 and 0.5 mug/ml, which is far below those values expected to cause a membrane-stabilizing or quinidine-like effect. The clinical applications of the results are discussed.