The adrenal gland combines essential components of the autonomic nervous system and the HPA axis in close contact. From morphological analyses employing immunohistochemistry, in situ hybridization, the novel technique of laser capture microdissection, and electron microscopy, it has been has shown that the chromaffin cells of the adrenal medulla and the steroid-producing cells of the adrenal cortex are extensively intermingled and functionally interrelated. In in vitro studies, a variety of regulatory factors produced and released by the adrenal medulla were identified as playing an important role in modulating adrenocortical function. An isolated adrenocortical cell deprived of its tissue integrity, input from the nervous system, or intercellular communication with chromaffin, vascular, and immune cells of the adrenal gland, loses its normal capacity to produce glucocorticoids and to adequately respond to the homeostatic challenges of stress. Adrenocortical cells in co-culture with chromaffin cells produced ten times more glucocorticoids than the same number of pure adrenocortical cells and demonstrated marked up-regulation in the mRNA expression of cytochrome p450 enzymes and STAR in the co-culture, while this expression was down-regulated in isolated cells. Transgenic animal models of over-expression or deletion of enzymes involved in catecholamine synthesis, as well as of altered function of components of the HPA axis, provide evidence that the mutual interdependence of the sympatho-adrenal system and the HPA axis at the level of the adrenal gland is of physiologic relevance in vivo. Alterations in intercellular communications, local production of neuropeptides, growth factors and cytokines, and aberrant expression of ectopic receptors on adrenal cells have been implicated in adrenal cell growth, development, hyperplasia, tumor formation, autonomous hormone production, and autoimmune disease. Moreover, we described a direct cellular interaction of lymphocytes with adrenal cells as a novel non-cytokine mediated mechanism of immune endocrine interactions. Highlighting the importance of the extra-pituitary mechanisms of adrenocortical regulation, be them neural or immune, is a worthwhile starting point for a more complete analysis of the human stress system in vivo.