The purpose of this study was to examine whether young adults changed their physical activity (PA) behavior when wearing motion sensors. PA patterns of 119 young adults (M age = 20.82 years, SD = 1.50, M body mass index = 23.93 kg/m[superscript 2] , SD = 4.05) were assessed during 2 consecutive weeks. In Week 1, participants wore an accelerometer. During Week 2, participants wore an accelerometer and a pedometer. Counts/day from the accelerometer were examined over the entire study period. Steps/day from the pedometer were examined during Week 2. Descriptive statistics and two-factor repeated-measures analysis of variance were calculated. There were significant differences in counts/day by study day during both weeks of the study (Week 1, p less than 0.0001; Week 2, p less than 0.0001), and steps/day during Week 2 (p less than 0.0001). However, reactivity was not evident. Subsequent investigation by day of the week revealed these differences were due to changes in participants' PA behavior by weekdays and weekends. With respect to gender, men were more active than women during both weeks of monitoring, but daily PA behavior between the genders followed a similar pattern. In conclusion, reactivity to accelerometers and pedometers was not evident in this sample. (Contains 2 tables and 2 figures.)