IMPLANTABLE CARDIAC PACEMAKERS are being used with increasing frequency in the management of patients with symptoms of a complete heart block. Although electromechanical control of heart rate would seem to demand a high level of both knowledge and professional concern regarding interactions between pacemakers and the environment, little information is available in scientific journals. We are, therefore, reporting preliminary observations of the behavior of two brands of pacemakers in several environments which may be encountered by patients. Material and Methods Three pacemakers have been used. Two were designed and made by one of us (R. W. S.) and are of the type used in 26 patients treated in this institution.1,2 One of these had the long lead wires intact, the other had them removed to ensure that any induction currents or alterations of function which were detected had originated within the pacemaker unit itself. The third pacemaker, a popular,