This study investigates the relationships between aggression and rejection/neglect by mothers, child temperament, day care experience for preschoolers; in addition, the moderating effect of self-regulation was investigated. The subjects were 338 preschool children. Teachers completed questionnaires that assessed aggression and self-regulation. Mothers reported on the temperament of children and parenting behavior. The rejection/neglect by mothers, child temperament, and daycare experiences were significant predictors of aggression. The moderating effect of self-regulation was found only in daycare experience, which suggests that high self-regulation protects aggression from day care experience.