1 The effects of verapamil (0.15/kg) and fendiline (3 mg/kg) were studied in anaesthetized, thoracotomised dogs with a critical constriction of the left anterior descending coronary artery, paced in excess of the initial rate by 60-70 beats/min. Epicardial ST-segment elevation and changes in lactate uptake were used to assess the severity of myocardial ischaemia. 2 Both drugs prevented the ST-segment elevation and the reduced lactate uptake that resulted from atrial pacing. 3 The anti-ischaemic effect of fendiline is mainly due to its negative chronotropic action, whereas that of verapamil is due in part to bradycardia and in part to the reduced preload and afterload. In addition, both agents increase coronary flow to the ischaemic area and thus improve the myocardial oxygen supply/oxygen requirement ratio.