In a case of pacemaker failure, insertion of a second transvenous electrode into the right ventricle resulted in apparent immediate pulmonary embolism and simultaneous restoration of effective stimulation by the original malfunctioning electrode. The case provided an exceptional opportunity to recognize an offending perielectrode thrombus and to time precisely the sequence of events after pulmonary embolism. The effectiveness of anticoagulant therapy in preventing electrode interference by thrombosis was suggested. © 1969.