The effects of the new rate-responsive pacemaker Medtronic Activitrax on maximal exercise heart rate and physical work capacity were studied acutely and after two weeks in 10 patients. Compared to the VVI-mode, heart rate increased in activity mode during exercise from 102.1 +/- 29.0 to 115.8 +/- 13.5/min (p less than 0.05). Similarly, physical work capacity increased from 343 +/- 260 to 528 +/- 354 watts X min (p less than 0.02). This effect persisted over 2 weeks. Thus, using rate-responsive pacing a significant improvement in working capacity could be shown in patients with inadequate exercise heart rate.