Propranolol caused a reduction of left circumflex coronary blood flow and increased coronary resistance in open-chest anaesthetized dogs. Similar results were shown at rest and during exercise produced by repetitive electrical stimulation of the leg muscles. Diminished coronary flow occurred in association with diminished left ventricular work and a diminished product of arterial mean pressure and heart rate. This suggests that myocardial oxygen consumption decreased and may have been responsible for the diminished coronary flow.