The pituitary—adrenal axis participates in the diurnal response of the organism to the environment and in its response to stress. Circulating glucocorticoid and mineralocorticoid hormones act on cells in the brain via Type I and Type II receptors, which operate at the level of gene regulation, and mediate various feedback effects of adrenal steroids on brain chemistry and structure, including the operation of second-messenger generating systems, levels of structural proteins of glial cells, and the death and survival of neurons. The hippocampus is prominent in these effects, and it also displays the highest level of Type I receptors of any brain region. New concepts point to a special role of the hippocampus, a nexus of cognition and emotion, in the feedback actions of adrenal steroids during the diurnal rhythm, and in response to stress. The hippocampus is, therefore, a prime target area for investigation of the events which accompany stress, and which may be related to the maladaptive state that results in depressive illness. Initial studies are described which investigate the effects of tianeptine on pituitary—adrenal function, hippocampal morphology and Type I and Type II receptor levels.