The goal of the present study was to test the hypothesis that, when children respond to peer provocation assertively, their physiology at that moment will be marked by high levels of respiratory sinus arrhythmia (RSA). Participants were 35 European American (66%), African American (27%), Latino American (3%), and mixed race/ethnicity (3%) children (16 girls and 19 boys; M[subscript age] = 11.35 years) from a Mid-Atlantic state. Children participated in a novel procedure in which they were provoked by and responded to a virtual peer while their RSA was assessed and behavioral responses were observationally coded. When RSA increased by one unit, children were about 17 times more likely to display at least one assertive response. These findings highlight the importance of RSA in children's calm, composed, and assertive responding to peer provocation, as well as the importance of linking children's behavior and physiology as they occur at the same moment.