Research has recognized that parental emotion regulation influences whether parents respond sensitively to their children in challenging parenting situations. However, parental emotion regulation is usually assessed using questionnaires that are not about parenting, rather than through examining parents' reaction to specific parenting situations that might evoke negative emotions. This study investigates individual differences in mothers' emotion regulation during parenting, specifically examining the relation between their subjective negative emotions and observed parenting behaviors and whether this relation is moderated by cognitive (strategies to manage negative emotions) and physiological (resting baseline and reactivity of respiratory sinus arrhythmia; RSA) processes. Data of 157 mothers' self-reported negative emotions and strategy-use, their RSA, observed maternal responsiveness, and their preschool-age children's (30-60 months, 49.7% female) challenging behaviors were collected during a Wait Task, in which mothers told children to wait before opening an appealing gift. Regression analysis indicated that, after controlling for how challenging children were, mothers' level of negative emotion was not associated with observed level of maternal responsiveness. In line with hypotheses, the association was moderated by mothers' resting RSA and the extent to which they suppressed negative emotions. However, contrary to hypotheses, the association was not moderated by use of reappraisal, distraction, or rumination, or RSA reactivity. The significant findings suggest that, although mothers' subjective experiences of negative emotions are not necessarily related to less responsive parenting behaviors, the link between maternal emotions and parenting behavior may indicate differences in how mothers engage cognitive strategies as well as their physiological regulation capacity. (PsycInfo Database Record (c) 2023 APA, all rights reserved).