Advances in neuropeptide neurobiology in the last decade are illustrated by studies of corticotropin-releasing factor (CRF), the 41 amino acid-containing peptide that controls the anterior pituitary secretion of adrenocorticotropin and other pro-opiomelanocortin products. Corticotropin-releasing factor is synthesized in both hypothalamic and extrahypothalamic perikarya in a large prohormone form, (186 amino acids), then it is processed and transported to nerve terminals where it is released in its active form by a calcium-dependent mechanism. Corticotropin-releasing factor biosynthesis can now be measured by in situ hybridization because of the elucidation of the CRF gene sequence. Once released, CRF acts on high-affinity CRF receptors, and signal transduction is mediated by activation of adenylate cyclase in certain brain areas, and perhaps by phosphoinositide hydrolysis. In other brain areas CRF is inactivated by peptidases that degrade the hormone, though these are not well characterized. A CRF binding protein has been identified in plasma, and perhaps in brain. Considerable evidence exists from cerebrospinal fluid studies, postmortem tissue receptor measurements, and CRF stimulation test studies to support the hypothesis that CRF is hypersecreted in depression, resulting in both pituitary-adrenal axis hyperactivity and certain signs and symptoms of depression, e.g., decreased libido, insomnia, and decreased appetite. There is also evidence for an involvement of CRF in the pathophysiology of anxiety disorders and in the mechanism of action of benzodiazepines. The development of selective CRF-receptor antagonists will permit direct testing of the hypothesis that CRF hypersecretion is responsible for certain of the cardinal features of affective and anxiety disorders.