This study examined the acute effects of a 10-min teacher-implemented classroom-based activity break (AB) on physical activity participation and time on-task in a preschool-age population. 118 ([mean] age = 3.80 +/- 0.69 years) students from one preschool served as participants. The intervention took place over 4 days: 2 days AB were conducted and 2 days typical instruction occurred. Physical activity was monitored via accelerometry and time on-task was measured by direct observation. Results demonstrated that AB led to a higher percent of moderate-to-vigorous physical activity (MVPA) during the AB ([mean] = 29.7%, p .001). Breaks also promoted more on-task behavior (F1,117 = 18.86, p .001) following the AB. Specifically, the most off-task students before the break improved on-task behavior by 30 percentage points (p .001). Percent of school day MVPA was also higher during AB days (t117 = 3.274, p = .001). Findings indicate teachers may improve time on-task postbreak for preschoolers with a short bout of physical activity in the classroom, especially in children who are the most off-task. In addition, classroom-based AB resulted in marginal increases in MVPA during breaks that influenced whole day activity. (author abstract)