Corticotropin releasing factor (CRF) is a 41 amino acid peptide first isolated from sheep hypothalami and thought to be a principal modulator of the hypothalamic-pituitary-adrenal cortical (HPA) axis. We report herein a series of clinical studies with CRF in healthy volunteers and in patients with abnormalities in HPA function, including depression, Cushing's disease, Cushing's syndrome, and Addison's disease. Our data indicate that CRF can be a diagnostic aid in distinguishing various disorders of the HPA axis from one another, including Cushing's disease from depression and secondary from tertiary adrenal insufficiency. Moreover, the hormone responses to CRF help clarify the pathophysiology of the HPA abnormalities in several disorders. For instance, our data indicate that hypercortisolism in Cushing's disease results principally from a defect at the level of the pituitary; in contrast, in depression the defect seems to be hypothalamic, possibly involving hypersecretion of endogenous CRF. This latter possibility is of particular interest in light of clinical observations that depression often can be precipitated by stress. Moreover, data from experimental animals show that CRF may influence several processes known to be altered in the overall symptom complex of depression, including not only pituitary-adrenal function, but also motor activity, appetite regulation and sexual behavior.