The present study examined developmental and gender differences in sensitivity to peer rejection across the transition into adolescence by examining beat-by-beat heart rate responses. Children between the ages of 8 and 14 years were presented with unfamiliar faces of age-matched peers and were asked to predict whether they would be liked by the other person. Their prediction was followed by feedback indicating that the peer had accepted or rejected them. Results revealed cardiac slowing to unexpected peer rejection in 11- to 14-year-olds. The cardiac response to unexpected rejection was most pronounced in girls. This pattern of findings supports the hypothesis of an increase in sensitivity to peer rejection after transition into adolescence and indicates that social rejection may be particularly salient in adolescent girls.