An important interaction between ß-adrenergic receptor (AR) stimulation and the myocardial contractile response to increased heart rate has only recently been identified. The effect of ß-AR stimulation to amplify the force-frequency effect is responsible for a large component of the positive inotropic effect of exercise in conscious dogs and also can be demonstrated at rest with dobutamine infusions over a range of paced heart rates. The interaction is apparent across species (dog, pig, mouse, rabbit, man). Amplification of the heart rate effect on myocardial contractility by dobutamine is lost in heart failure, which may play an important role in impaired exercise tolerance in that setting. In syndromes of chronotropic incompetence when adrenergic control is intact, it may be expected that a normal response of myocardial contractility to exercise will be achieved only when the interaction with heart rate is optimized by use of a rate-responsive cardiac pacemaker.