Changes in left diastolic filling which had been caused by treatment with the beta-adrenoblocker atenolol given in a dose of 50-100 mg/day were studied in 66 patients with Functional Class I-II exercise-induced angina pectoris. Echocardiography was performed during isometric exercise tests before and 7-9 days after drug administration. It was ascertained that not only ino- and chronotropic mechanisms, but redistribution in diastole patterns in the direction of increasing atrial systole were involved to adequately maintain cardiac output during exercise. Ejection fraction became higher in response to exercise with predominant myocardial mass, whereas atrial systolic fraction, with predominant dilatation. The drug produced its negative inotropic effect in the patients with predominant left ventricular dilation; a decrease in the rapid filling fraction was not followed by an increase in the atrial systolic fraction, which may be regarded as an early sign of myocardial failure.