Conventional doctrine states that the QT interval is related to heart rate in an inverse exponential relationship, so that with increasing rate the QT interval shortens. This relationship has recently been studied in a group of patients undergoing physiological exercise stress tests, atrial pacing stress test, and in a further group of patients with complete heart block undergoing exercise at a fixed ventricular rate controlled by cardiac pacemaker. Examinations of recordings made during physiological exercise do show the expected shortening in QT interval, we believe that this shortening is only in part due to the intrinsic effect of increased rate as patient who were atrially paced to similar rates and within the same age group showed only a small decrease in measured QT interval and patients undergoing exercise at fixed ventricular rate showed shortening in QT interval which was related to the independent atrial rate. It appears, therefore, that the QT interval is governed mainly by extrinsic factors and not intrinsically rate related. The physiological control of QT interval is being used now to construct a cardiac pacemaker which senses the interval between the delivered stimulus and the evoked T wave so that the stimulus-evoked T wave interval could be used to set the subsequent escape interval and subsequently the overall pacing rate. Physiological control of cardiac pacing rate using conventional unipolar lead systems and independent of atrial activity is possible and currently being investigated.