1. Changes in heart rate were evoked by atrial pacing in anaesthetized dogs with no pretreatment and in dogs given reserpine or guanethidine for 72 h. The effect of alterations in heart rate were related to two indices of myocardial contractility: the maximal rate of change of left ventricular pressure (dp/dt), and an index which was independent of initial fibre length (dp/dt)/IIT, where IIT is integrated isometric tension. 2. An increase in heart rate in control dogs was accompanied by a rise in both dp/dt and (dp/dt)/IIT confirming that the Bowditch staircase does exist in the intact ventricle. The regression line relating heart rate to (dp/dt)/IIT was significantly steeper than that relating heart rate to dp/dt because the reduction in left ventricular preload at high heart rate tends to attenuate the rise in dp/dt. 3. Reserpine, but not guanethidine pretreatment was accompanied by either a slight decrease or no change in (dp/dt)/IIT during pacing. 4. Acute elevation of (dp/dt)/IIT by either calcium or isoprenaline infusion in reserpine pretreated dogs did not restore the Bowditch effect. 5. Acute depression of (dp/dt)/IIT by propranolol and pentobarbitone was accompanied by a greater rise in (dp/dt)/IIT with pacing in control dogs and a rise rather than a fall in reserpine-pretreated dogs.