Purpose: This study sought to determine how children's participation in physical activity during a mastery-motivational climate changed during a 20-week intervention and to compare it to children's free-play activity during a typical day at their local day-care facility. Method: Twelve 4-year-old children participated in a mastery-motivational climate physical activity program delivered 2 days a week for 20 weeks during a period of 8 months. All children were fitted with an Actigraph GT3X triaxial accelerometer. Data from the accelerometers were reduced to determine minutes of sedentary time, light physical activity, and moderate-to-vigorous physical activity (MVPA) during (a) all 20 weeks, and (b) Weeks 1 through 4, Weeks 9 through 12, and Weeks 17 to 20 for the mastery climate. Activity levels in the mastery condition were compared to the children's activity during unplanned free play in Weeks 1, 10, and 20. Results: During the course of the mastery-motivational climate program, participation in sedentary behavior decreased statistically significantly, while participation in MVPA increased statistically significantly. Within the free-play condition, there were no changes in the levels of activity across time, with the children spending on average more than 80% of their time being sedentary. Conclusions: Findings indicate that when specific task structures associated with a mastery climate are included in an instructional setting, these climate manipulations seem to have a direct effect on physical activity levels once the children learned how to manage themselves in the setting. Free-play activity in and of itself does not appear to stimulate MVPA.