The current study examined associations between mothers’ mental representations of attachment and mothers’ and infants’ physiological response to a moderate stressor. Mothers’ and their 4-month-old infants’ (N = 33) respiratory sinus arrhythmia (RSA) were assessed during a modified Still-Face Paradigm. Mothers’ mental representations of attachment were assessed using the Adult Attachment Interview. Consistent with prior research, infants’ RSA generally decreased during still-face episodes and returned to baseline when mothers resumed interaction. Mothers’ RSA increased during Still-Face and decreased at Reunion. Mothers’ RSA explained unique variance in infants’ RSA at some episodes. Infants of Dismissing mothers showed higher RSA during the second Still-Face Reunion sequence than infants of Autonomous mothers, and Dismissing mothers’ RSA decreased at second Reunion. Although not statistically significant due to small cells, sizeable effect sizes suggest that infants’ and mothers’ RSA across the still-face procedure may differ as a function of mothers’ state of mind regarding attachment.