A noninvasive procedure has been developed to assess effectiveness of activity-sensing rate response ventricular (VVIR) pacing systems in patients with implanted conventional fixed-rate ventricular (VVI) pacemakers capable of triggered mode operation (VVT). A VVIR pacemaker (activity threshold medium or low, rate response 6 to 10, upper rate 125 or 150 beats/min) was strapped to the chest wall (prepectoral area) of patients with previously implanted fixed-rate VVI pacemakers. In 17 of the 18 patients included in the study, the implanted pacemaker was capable of triggered mode operation (VVT). Triggering of the conventional pacemaker was achieved by reprogramming it to the VVT mode (high sensitivity, short refractory) and connecting the output of the "strapped-on" VVIR pacemaker (5 volts, 1.5-ms pulse width) to 2 standard cutaneous electrodes positioned so as to be in proximity to the implanted intracardiac electrode system. Patients underwent symptom-limited treadmill exercise tests during both VVI and VVIR pacing. Triggering conventional VVI pulse generators by a "strapped-on" VVIR pacing system proved feasible in 16 of 17 cases and improved overall heart rate response (fixed rate 86 ± 22 vs VVIR 118 ± 7 beats/min) and exercise duration (fixed rate 6.6 ± 4.9 vs VVIR 10.1 ± 4.8 minutes) (mean values for all patients studied during exercise testing). Triggering of a previously implanted permanent pacemaker by a strapped-on activitytriggered device may be useful to assess rate response pacing before implantation of a VVIR device in patients in whom elective pulse generator replacement is planned.