One of the responses of an animal to injury, infection or other stresses is the release from the adrenal cortex of glucocorticosteroids(GCs). In humans this is predominantly hydrocortisone (cortisol), but other animals may release different but related steroids. For a long while it was believed that steroids were in some way involved in the mounting of the response to stress (such as an inflammatory response), but it has only recently become clear that in fact steroids are actually part of system which prevents our ‘defencemechanisms’ from running out of control and threatening homesostasis. Glucocorticosteroids are thus part of the normal regulation of the immune system, which explains why adrenalectomized animals tend to produce exaggerated responses to environmental trauma such as infection or injyry.