Research documents bidirectional associations between parental overcontrol (OC) and children's anxiety; OC may place children at risk for anxiety and also may occur in response to children's requests for help. However, to date no studies have examined children's or parents' in-the-moment emotional responses to OC. Using a community sample of mothers and school-age children, we examine the individual and interactive influences of maternal OC, maternal anxiety, children's help-seeking, and children's anxiety in predicting physiological reactivity in response to a stressor faced by children and observed by mothers, predicting that for children of higher anxiety mothers, higher OC will be associated with increases in reactivity (decreases in respiratory sinus arrhythmia [RSA]), whereas for higher anxiety mothers themselves, engaging in OC will be associated with reductions in physiological reactivity (decreases in heart rate). Multilevel modeling suggested that for children of higher anxiety mothers, greater peak OC is associated with greater reductions in RSA (increases in reactivity) after the onset of OC. In contrast, for higher anxiety mothers themselves, greater peak OC was linked with attenuations in heart rate. Effects held when controlling for children's anxiety and help-seeking, and no pattern of effects was observed with analyses in which children's help-seeking was the predictor or children's anxiety was the moderator, suggesting that in this case, physiological reactivity is uniquely associated with the interaction between maternal OC and anxiety. Among mothers with higher anxiety, OC may serve a regulatory function, reducing physiological reactivity, while exacerbating children's reactivity.