Abstract A method for estimating myocardial threshold in patients with implanted artificial pacemakers has been described. By the external application of a calibrated suppression signal, the output of the implanted unit can be gradually reduced until the myocardial response ceases. The threshold level thus determined can be expressed as a per cent of available pulse energy by a simple calculation. This method promises to have clinical application because it is relatively simple and entirely external. It has been demonstrated to be safe, no sequelae having occurred during over 700 human studies. In combination with our previously reported method of estimating interelectrode impedance, it should provide a rational means of serially evaluating pacemaker function following implantation. It is applicable to both direct myocardial and transvenous modes of pacing, but its most important limitation at the present time is that it can be used with only one of the commercially available pacemakers. By application of quantitative threshold measurements, we have been able to confirm our clinical observations of exit block as a significant cause of pacemaker failure. This complication comprised the second most common cause of failure to pace, 14 per cent in a series of 128 implantations. It has also been possible by this means to verify the effectiveness of glucocorticoid and sympathomimetic drugs in lowering the myocardial threshold level. The implications of these observations have been briefly discussed. Threshold analysis may prove to be an important means of investigating the myocardial effects of other pharmacologic preparations in the future.