Eleven patients with rate responsive pacemakers (7 men, 4 women, mean age 41 years with a range of 23-60) were randomly assigned to a cross-over study in order to assess their overall exercise capacity and quality-of-life (QOL) scores. All of the pacemakers were implanted for complete AV block or sick sinus syndrome. The pacemakers were randomly programmed into VVI or rate responsive (VVIR) pacing modes for 3-week study periods in each mode. At the end of each period, an exercise test was performed and the QOL was evaluated by the "Hacettepe Quality-of-Life Questionnaire". All patients exercised longer in the VVIR mode (mean 10.54 +/- 0.73 min) than in the VVI mode (mean 7.81 +/- 0.62 min) (P less than 0.05). QOL scores were also found to be significantly higher in the VVIR mode (mean 173.81 +/- 16.22 points) compared to the VVI mode (mean 156.27 +/- 21.22 points) (P less than 0.01). In conclusion, our results suggest that VVIR pacing offers a better QOL in addition to an improved exercise capacity, compared to the single chamber nonrate modulated pacing (VVI).