This thesis is an investigation of how children's cardiac reactivity differed between a baseline and novel stressor episode as a function of their Secure vs. Insecure-Ambivalent attachment status. Children's physiological regulation was measured in response to the novel stressor, an 'Interesting-but-Scary ' (IbS) mask that spoke to the child and asked him or her to approach. Assessment at 3.5 years ('N'=48) included heart rate and vagal tone during a baseline episode and the IbS episode. Security of attachment was assessed in the modified Preschool Strange Situation Paradigm. Children only showed meaningful physiological regulation (i.e., vagal withdrawal) to the stressor if they were classified as Secure, not Ambivalent. This finding remained significant even when examined only within children who coped successfully with task demands by approaching the IbS mask. Findings are discussed in relation to children's emerging physiological and emotion regulation skills in the context of their attachment relationship.