Inhibition of demand pacemakers by interference arising from myopotentials has not received due recognition as a significant clinical problem. We have encountered four cases in which cerebral ischemic episodes resulted from pacemaker inhibition associated with isometric contraction of the subadjacent pectoralis major. It can be shown that even myoelectric signals detected on the skin surface are capable of inhibiting implantable ventricular-inhibited units. In a sample of our patients, 24% were shown to be vulnerable to pectoral muscle contraction by routine electrocardiographic testing. The problem may be very difficult to eliminate since significant and even maximal myoelectric energy levels may occur at frequencies within the QRS spectrum.