The effect of rapid pacing on the atrial constant voltage stimulation threshold in humans has not been defined at rates applicable to those of antitachycardia pacing. The effect of pacing rate on the atrial strength-duration relation was determined in 10 patients at pacing rates between 125 and 300 beatstmin to explore excitability over the range of rates used for permanent antitachycardia pacing systems. Two points that define the strength-duration curve were measured at each pacing rate: rheobase voltage-the lowest stimulus voltage that results in capture at a pulse duration of 2 ms; and chronaxie pulse duration-the threshold pulse duration at twice rheobase voltage. A permanent, tined, J-shaped pacing lead with a high current density and low polarization electrode was positioned in the right atrial appendage for cathodal stimulation. A constant voltage output, incorporating a fast recharge pulse designed to minimize electrode polarization, was used for stimulation. There was a significant increase in rheobase voltage (p = 0.009), chronaxie pulse duration (p = 0.001) and minimal threshold stimulus energy (p = 0.05) at pacing rates >225 beats/min. A rheobase voltage >5 V occurred in three patients at pacing rates ≥275 beats/min. At a pacing rate of 300 beats/min, rheobase voltage had increased in 8 of 10 patients. The atrial effective refractory period determined at 1.5 times late diastolic threshold and a cycle length of 500 ms demonstrated a modest correlation with rheobase voltage at pacing rates ≤225 beats/min (r = 0.8), but not at ≥250 beats/min (r = 0.36). There was no correlation between either rheobase voltage or chronaxie pulse duration and atrial effective refractory periods determined at three times late diastolic threshold at any pacing rate. These data demonstrate that the human atrial strength-duration curve is shifted upward and to the right at pacing rates applicable to antitachycardia pacing, and have important implications for the design and programming of permanent antitachycardia pacing systems. © 1990.