Almost any kind of threat to homeostasis or stress will cause plasma glucocorticoid levels to rise. The increased levels have traditionally been ascribed the physiological function of enhancing the organism's resistance to stress, a role well recognized in glucocorticoid therapy. How the known physiological and pharmacological effects of glucocorticoids might accomplish this function, however, remains a mystery. A generalization that is beginning to emerge is that many of these effects may be secondary to modulation by glucocorticoids of the actions of numerous intercellular mediators, including established hormones, prostaglandins and other arachidonic acid metabolites, certain secreted neutral proteinases, lymphokines, and a variety of bioactive peptides. These mediators participate in physiological mechanisms--endocrine, renal, immune, neural, etc.--that mount a first line of defense against such challenges to homeostasis as hemorrhage, metabolic disturbances, infection, anxiety, and others. Contrary to the traditional view that glucocorticoids enhance these defense mechanisms, however, it has become increasingly clear that glucocorticoids at moderate to high levels generally suppress them. This paradox, which first emerged when glucocorticoids were discovered to be antiinflammatory agents, remains a major obstacle to a unified picture of glucocorticoid function. We propose that stress-induced increases in glucocorticoid levels protect not against the source of stress itself but rather against the body's normal reactions to stress, preventing those reactions from overshooting and themselves threatening homeostasis. This hypothesis, the seeds of which are to be found in many discussions of particular glucocorticoid effects, immediately accounts for the paradox noted above. Furthermore, it provides glucocorticoid physiology with a unified conceptual framework that can accommodate such apparently unrelated physiological and pharmacological effects as those on carbohydrate metabolism, inflammatory processes, shock, and water balance. It also leads us to suggest that some of the enzymes rapidly induced by glucocorticoids, such as glutamine synthetase, detoxify mediators released during stress-induced activation of primary defense mechanisms. These mediators would themselves lead to tissue damage if left unchecked.