Mifepristone is a selective type II glucocorticoid receptor antagonist with a favorable safety profile. It binds to the same site as the synthetic glucocorticoid dexamethasone and blocks the negative feedback control of cortisol on the pituitary. Thus, mifepristone, because it directly antagonizes the glucocorticoid receptor, which has been found to be more sensitive in PTSD in several models, is ideally suited for use in determining the pathophysiological significance of increased glucocorticoid receptor sensitivity in PTSD and whether its attenuation is of therapeutic value.