Experiencing maltreatment in early childhood predicts poor parasympathetic regulation, characterized by low baseline parasympathetic activity and strong withdrawal of parasympathetic influence in response to tasks. The Promoting First Relationships^® (PFR) program improves parental sensitivity toward young children in families identified as maltreating. Using a subsample from a randomized control trial, we examined whether parental participation in PFR had lasting effects on toddlers' parasympathetic regulation, as measured by respiratory sinus arrhythmia (RSA), relative to a resource and referral control condition. In addition, we examined whether parental sensitive and responsive behavior mediated or moderated associations between parent treatment group and children's RSA. More than 6 months after completing treatment, 29 families in the PFR condition and 30 families in the control condition were visited at home, and toddlers' RSA was assessed at baseline and during five moderately challenging tasks. Groups did not differ in baseline RSA, but differed in RSA reactivity to the tasks. Across tasks, toddlers of parents in the control condition manifested significantly larger RSA decreases than toddlers of parents in the PFR condition. Parental behavior showed divergent associations with RSA change for toddlers of parents in the PFR versus control condition, with PFR treatment predicting RSA change ranging from small decreases to increases in toddlers of parents who showed the most sensitive, responsive behavior in the 6 months following treatment. This preliminary study showed that the same intervention that improved parenting also improved toddlers' parasympathetic regulation in response to everyday activities, warranting further experimental investigation.