Objective: To determine whether foster children showed different autonomic nervous system activity on separation and reunion than control children. Autonomic nervous system activity in foster children was examined in relation to time in placement and disinhibited attachment. Method: The sample included 60 foster and 50 control children between 2 and 7 years of age who participated with their caregivers in a modified Strange Situation. Heart rate, respiratory sinus arrhythmia (RSA), and pre-ejection period were monitored continuously. Foster caregivers reported disinhibited symptoms on the Disturbances of Attachment Interview. Results: The Strange Situation elicited less RSA reactivity in foster children. Differences in RSA, heart rate, and pre-ejection period responses on the specific separation and reunion episodes were not significant. RSA responses on separation from the stranger and on reunion with the foster caregiver were partly explained by time in placement and disinhibited attachment. Conclusions: Early experiences of relationship disruptions in foster children as well as short placements may have an impact on children's adaptation to environmental and relational challenges. Stable placement may facilitate adaptive affect regulation, except for children with disinhibited symptoms. (Contains 1 figure and 1 table.)