In this study, physical activity (PA) was objectively measured in 213 Belgian preschoolers (M(age) = 4.98, SD = .88 years) over 4 consecutive days including two weekend days. Within-day variability in PA showed a typical activity pattern during weekdays and weekend days. Weekdays clearly reflected a preschool attending day with more peaks and troughs than weekend days and after-school hours were characterized by a decrease in activity. Between-day variability in PA was identified in preschool girls above the age of four, suggesting that the lack of a structured preschool environment is already related with a decrease in PA in this sex-specific age group. The results of this study are informative for the development of future PA interventions and indicate that both the preschool and the home environment should be targeted in the promotion of preschoolers' PA.