The role of the mother-toddler attachment relationship in moderating the relations between behavioral inhibition and changes in salivary cortisol levels in response to novel events was examined in 77 18-month-olds. Behavioral inhibition was determined by observing toddler inhibition of approach to several novel events. Attachment security to mother was assessed using the Ainsworth Strange Situation. Changes in salivary cortisol were used to index activity of the stress-sensitive hypothalamic-pituitary-adrenocortical (HPA) system. In addition, toddler coping behaviors and the behaviors used by mothers to help toddlers manage novel events were examined. Elevations in cortisol were found only for inhibited toddlers in insecure attachment relationships. Mothers in these relationships appeared to interfere with their toddlers' coping efforts. These results are discussed in the context of a coping model of the relations between temperament and stress reactivity.