The pacing threshold of the human heart may be altered by physiological factors such as physical exercise. These changes may influence the individual programming of a pacemaker, since pacemakers can be programmed at pulse amplitudes of 2.5 volts and less. We investigated 22 patients with a multiprogrammable ventricular demand pacemaker 3 months after implantation; 16 patients had received a steroid-eluting lead and six patients had an Elgiloy lead. Parameters measured at rest and immediately after exercise were: Voltage threshold at pulse durations between 0.05 and 0.6 ms, impedance, R wave amplitude and energy consumption for the pacing threshold at 0.5 ms pulse duration. All patients performed a symptom limited supine bicycle exercise test. None of the investigated parameters showed a significant difference between rest and exercise, neither for the steroid eluting lead nor for the Elgiloy lead. The data suggest that the individual programming of a pacemaker adapted to the measurements at rest is also reliable and safe during exercise.