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Objective: This study examined the unique and interactive effects of PA volume and within-person fluctuations in PA volume (i.e., intraindividual variability in PA volume; PA-var) on preschoolers' (N = 141; 47.5% girls) ADHD symptoms. Method: Preschoolers wore accelerometers during the school day over a 2-week period. Teachers reported on children's hyperactive, impulsive, and inattentive symptoms on the ADHD Rating Scale--IV Preschool Version. Results: In the context of regression models, higher levels of PA-var were linked with lower levels of impulsive symptoms. Higher levels of PA volume were linked with higher levels of hyperactive and inattentive symptoms, but only when PA fluctuation (i.e., PAvar) was relatively low. Post hoc analyses with teacher-reported impairment as the outcome revealed a significant interaction such that the positive association between PA volume and impairment was stronger at lower, as compared to higher, levels of PA-var. Conclusion: Larger fluctuations in preschoolers' PA volume may indicate lower risk for displaying impulsive symptoms. Moreover, preschoolers with high levels of PA that remain relatively consistent throughout the day may be at increased risk for exhibiting hyperactive and inattentive symptoms and related impairment, suggesting they are unable to regulate their activity to meet expectations in the school environment.