Numerous evidences suggest that early life events can affect the development of the nervous system, contributing in shaping interindividual differences in vulnerability to stress or psychopathology. A number of studies have shown that mothering style in rodents can produce neuroendocrine, neurochemical, and behavioral changes in the adult, although the basic mechanisms initiating this cascade of events still need to be investigated. This paper reviews research performed in our and other laboratories investigating some of the features characterizing hypothalamic--pituitary--adrenal (HPA) axis activity of rodents during early development, with a special emphasis on extrinsic, social regulatory factors, such as the mother and the siblings. In addition, a possible role for neurotrophins as mediators of the effects of external manipulations on brain development is suggested.