Exercise performance after implantation of the pacemaker with physical activity dependent pacing rate (Activitrax, Medtronic) was assessed by treadmill tests one month afterwards in 10 patients and a further 6 months later in 6 of them, who had long been VVI pacing dependent (reimplantation subgroup). In the 1 month test exercise performance had improved in 7 of 10 in activity pacing mode compared to VVI mode. After 6 months of physical activity dependent pacing the patients in the reimplantation subgroup showed an improved exercise test in VVI mode compared to the 1 month test in VVI mode. In activity pacing mode there was an additional improvement (increase of exercise duration from 12.1 +/- 2.5 to 14.5 +/- 2.7 min, p less than 0.005, and of accumulated work from 31.1 +/- 15.1%, p less than 0.002) compared to the VVI mode. Since Activitrax PM allows better exercise performance at longterm follow-up, it becomes a pacing mode of choice when double chamber pacing is impossible.