The purpose of this paper is two fold. First, to revisit the issue of the definition of stress and to highlight the difficulties with the contemporary definitions and, second, to review the literature on the influence of early experiences on the endocrine stress responses and behavior in rodents, sub-human primates and humans. Early experiences, usually involving some manipulation that results in disruption of the mother-infant relationship, have been shown to have long-term influences on the behavioral and endocrine responses to stress. In the rodent, brief periods of separation result in an attenuated adrenal response to stress (reduced secretion of corticosterone). In contrast, longer periods of separation result in an exaggerated response and several behavioral anomalies i.e. increased alcohol consumption, increased startle response etc. However, the effects of disruptions of the mother-infant relationships in primates reveal a pattern of behavioral disturbance but little influence on the endocrine response. Brief maternal separations result in a blunted cortisol response in juvenile squirrel monkeys. The long-term effects of early experiences in humans are very difficult to interpret. It is not possible to determine the length and severity of the experiences, and when in development the experiences were imposed on the child. Despite these limitations, there is a general consensus that adverse early experiences contribute to adult psychopathology.