The recent preparation of a purified pituitary adrenocorticotropic hormone in sufficient quantity for use in human subjects1has provided an opportunity to stimulate directly the human adrenal cortex.2It appeared likely that, in patients suffering from primary adrenal cortical insufficiency, the normal response to this specific stimulus would fail to occur. This expectation has been substantiated by clinical studies.3The development of a simple test for adrenal cortical insufficiency based on the failure of patients with Addison's disease to secrete cortical hormones in response to the pituitary adrenocorticotropic hormone has been the aim of this study. Among the numerous metabolic3and hematologic4changes which follow administration of the hormone in subjects with an intact adrenal cortex, two have been selected as the basis for a clinical test for adrenal cortical reserve because of their simplicity of determination and magnitude of change: (1) a fall in