Parents' physiological regulation may support infants' regulation. Mothers (N=152) and 6-month-old male and female infants were observed in normal and disrupted social interaction. Affect was coded at 1-s intervals and vagal tone measured as respiratory sinus arrhythmia (RSA). Maternal sensitivity was assessed in free play. Mothers and infants showed opposite patterns of RSA change. During disrupted interaction, mothers' RSA increased and infants' decreased, suggesting self-regulation of distress. During reunion, although the typical pattern was for infants to return to baseline levels, infants of sensitive mothers and sensitive mothers both showed a significant decrease in RSA from baseline. Mothers' and infants' physiological responses may be a function of mutual responsiveness.