T HIS SHORT review will describe mechanisms by which corticosteroid modification of cellular phospholipids and particularly the phospholipid-structure of a variety of membranes can produce some of the effects of the corticosteroids. In addition to these effects, many studies have demonstrated effects of other hormones to alter specific phospholipids, phospholipases, or for phospholipase action on cells to mimic hormone action. All actions of the corticosteroids are not thought to be produced by these lipid alterations, but such changes are capable of producing widespread effects on receptor activity, RNA and protein synthesis, enzymatic activity, and transport through membranes. The availability of new data indicating that phospholipids are altered considerably by corticosteroids in a number of tissues makes a review of this subject of interest at this time.Corticosteroids are secreted in response to a variety of "stresses" that represent alterations in the cellular environment that might result in death or injury to the exposed cells if proper adaptation did not occur. Thus, these hormones are increased in response to anoxia, very low or very high temperature, or hypoglycemia. Through mechanisms that have not been clearly understood the cell is protected by the increased hormones against potentially harmful alterations in its environment. Unicellular organisms, which lack a sophisticated endocrine system for response to stresses, undergo modifications in cellular membranes in response to stress. A decrease in the temperature at which the organism is grown would produce a decrease in the fluidity of the membrane with resulting marked effects upon membrane-bound enzymatic activities. Similarly, increased temperature would result in increased fluidity of the lipid membrane and similar alterations in its function. In order