This proposal describes studies of the development of behavioral and hormonal responses to stressors during infancy and early childhood. In addition to a longitudinal study (prenatal period to age 3), a large study of children responding to the challenges of preschool, and several smaller studies are also proposed. This research is organized around examination of the hypothalamic-pituitary-adrenocortical (HPA) system, a neuroendocrine system central to mammalian stress resistance. A major goal of this work is to integrate studies of the psychobiology of this system with research on the biological substrate of temperament in children. To this end, the studies described examine components of a proposed temperament-coping resources-physiological stress model. According to this model, the biological substrate of temperament (assessed through measures of baseline EEG asymmetry, vagal tone, basal HPA activity, and positive late components of the ERP) influences the child's likelihood of perceiving events as potentially threatening. However, activation of physiological stress reactions are presumed to follow from expectations that the threat will be realized. Mediating these perceptions are the coping resources available to the child. The studies will take advantage of both lab-based and naturalistic-ecological stressors and challenging situations. The proposed studies are designed to integrate 3 major literatures: (1) the physiology of infant/child temperament, (2) socioemotional development, including attachment research, and (3) psychoendocrine studies of infant/child stress. Finally, preliminary investigations of a noninvasive measure of immune functioning (salivary interleukin-1b) are also proposed that would extend this integration to research on individual differences in children's physical health and susceptibility to illness.