The main purpose of this study was to investigate whether rehearsal, defined as the tendency to recurrently ruminate over upsetting aversive experiences, had an effect on pedometry reactivity. A total of 156 Hong Kong Chinese children aged 9–12 years were recruited. Participants completed the Rehearsal Scale for Children‐Chinese (RSC‐C; Ling, Maxwell, Masters, & McManus, 2010) and wore the pedometers for 3 consecutive weeks. The mean number of steps was significantly higher in Week 1 than in Week 3. High rehearsers showed a larger decrease in mean number of steps from Week 1 to Week 3 than low rehearsers. Future physical activity intervention studies should adjust for reactivity in their baseline measurements and should further examine the relationship between habitual PA and individual propensities for rehearsal. © 2010 Wiley Periodicals, Inc. J Clin Psychol 00:1–6, 2010.