 1999 John Wiley & Sons, Inc. CCC 1012-1630/99/030188-09 Elysia Poggi Davis Bonny Donzella William K. Krueger Megan R. Gunnar Institute of Child Development University of Minnesota Minneapolis, MN 55455 The Start of a New School Year: Individual Differences in Salivary Cortisol Response in Relation to Child Temperament Received 3 March 1999; accepted 7 April 1999 ABSTRACT: Noon and evening salivary cortisol levels were examined in 70 elementary school children during the 1st week of a new school year. Samples were obtained on the 1st and 5th days of school and on weekend days. Delta cortisol scores were created to measure the change in children’s levels on initial school days relative to weekend days. Temperament was assessed using Rothbart’s Child Behavior Questionnaire, a parent report instrument. The three dimensions of surgency or extroversion, negative affectivity, and effortful control were examined. Positive correlations were obtained with Day 1 delta cortisol for negative affectivity and Day 5 delta cortisol for surgency. Contrary to the expectation that internalizing aspects of temperament (shyness, fearfulness) would be associated with larger increases in cortisol to the novelty and challenge of a new school year, these data indicate that larger increases in cortisol were observed in more extroverted children.  1999 John Wiley & Sons, Inc. Dev Psy- chobiol 35: 188–196, 1999 Keywords: temperament; children; cortisol Children respond differently to the social challenges in their life. Their ability to cope with social challenge depends on many factors, including temperament and experience. These factors are related to children’s be- havioral and physiological responses to social chal- lenges (Stansbury & Gunnar, 1994). Cortisol, a mea- sure of the activity of the hypothalamic – pituitary – adrenocortical (HPA) system, has been a focus of Correspondence to: M. R. Gunnar Contract grant sponsor: National Institutes of Health Contract grant number: MH 56958 Contract grant sponsor: National Institute of Mental Health Contract grant number: MH00946 many physiological studies of stress and challenge (Hennessy & Levine, 1979; Mason, 1975; Rose, 1980). The start of a new school year is a challenging event that almost every child must face. Entry into a novel social group is related increased activity of the HPA axis in studies with animals (Goo & Sassenrath, 1980; Lyons, Mendoza, & Mason, 1994; Saltzman, Mendoza, & Mason, 1991). In the following study, we examined whether the beginning of a new school year would stimulate increases in cortisol in human chil- dren. Studies of both adults and children have shown that cortisol rises in response to threatening or challenging