--- - |2 Resting respiratory sinus arrhythmia (RSA) may confer infant susceptibility to the postpartum environment. Among infants with higher RSA, there may be a positive relation between depressive symptoms across the first 6 months postpartum (PPD) and later behavior problems, and toddlers’ dysregulation during mother–child interactions may partially explain the effects. Among a sample of low‐income Mexican‐American families, infant RSA (N = 322; 46% male) was assessed at 6 weeks of age; mothers (Mage = 27.8, SD = 6.5) reported PPD symptoms every 3 weeks from 6 to 24 weeks and infant behavior problems at 36 months. Dysregulation was observed at 24 months. PPD was positively associated with behavior problems only among infants with lower RSA; however, this relation was not mediated by dysregulation. - Child Development, EarlyView.