BACKGROUND: We examined the effects of short bouts of structured physical activity (SBS-PA) implemented within the classroom setting as part of designated gross-motor playtime on preschoolers PA. METHODS: Preschools were randomized to SBS-PA (centers, N=5; participants, N=141) or unstructured free playtime (UPA) (centers, N=5; participants, N=150). SBS-PA consisted of structured PA implemented in the classroom during the first 10 minutes of gross-motor playtime followed by 20 minutes of free playtime. UPA consisted of 30 minutes of unstructured free playtime. Teachers implemented both conditions for 5 days/week for 6 months. PA was assessed with accelerometers (preschool-day) and direct observation (30-minute sessions). Generalized linear mixed models were used to examine the impact of the intervention. RESULTS: Regarding the 30-minute sessions, significant group main effects were observed for intervals spent at light (p.001) and moderate-to-vigorous PA (MVPA, p.001). Regarding the preschool-day PA, significant group by visit interaction was observed for percent time spent in total preschool-day MVPA (F (2, 254) = 3.54, p =.03). Percent of time spent in MVPA significantly decreased in both groups at 3 months and at 6 months. CONCLUSION: SBS-PA can be implemented in classroom settings; however, further research is needed to examine its impact on preschoolers PA levels. (author abstract)