The success of an implanted cardiac pacemaker is dependent upon the establishment of a harmonious relationship between the artificial pacemaker and the human receiver. Failure of a pacemaker system may arise from an electronic or mechanical defect within the pacemaker, a physiologic problem, or from a poor relationship between the normal function of both. Such malfunctions may necessitate the repair, replacement, or repositioning of a pacemaker component or removal of a source of external interference. True pacemaker malfunctions must be clearly distinguished from pseudo-malfunctions, where there is no pacemaker system defect. These situations may, variously, require the repair of faulty testing equipment, reassessment of falsely interpreted test data, or treatment of the patient for an unrelated disorder. This paper outlines a system for investigating patients with suspected pacemaker malfunction. For simplicity, only malfunctions associated with the ventricular inhibited pacing system will be discussed.