Domestication may have changed fundamental aspects of the stress response, as has previously been found in adult chickens (Gallus gallus). Severe or chronic stress can lead to immediate and/or long term alterations on behaviour, as well as decreased efficiency of the immune system, decreased reproductive success and increased stress sensitivity. Similarly, short- and long-term phenotypic changes has been observed when exposing early postnatal individuals to stress. Previous work in chickens has demonstrated immediate behavioural changes after stressor exposure at an early age, and also a dampened HPA-axis response. The aim of this study is to present a coherent view of the postnatal development and reactivity of the HPA-axis during the first 3 weeks of life together with behavioural responses in a domestic layer breed and in its ancestral breed, the Red junglefowl. By measuring the recovery behaviour after the exposure to an acute stressor, combined with HPA-axis reactivity we aim to obtain a deeper understanding of the effects and underlying mechanisms behind the changes after an acute stressor exposure. The preliminary results show a significant change in baseline corticosterone as well as in HPA-axis reactivity to stress during the first 23 days of life in both Red junglefowl and in layers. Domestication effects were seen on baseline corticosterone levels on day 1 and 23, and a significant difference in HPA-axis reactivity was seen on day 23. So far, we conclude that the HPA-axis of the chicken is reactive from day one post hatch and that it is still maturing and stabilizing during several weeks, and that the development differs between the breeds.