The discovery of small and potent peptide antagonists of the corticotropin-releasing factor (CRF) receptor is described. Through the structure-activity relationship studies of 12-amino acid peptide corresponding to the C-terminal residues of astressin, we assumed that a particular surface of the alpha-helix was important for binding to the receptor. The small peptide containing d-Ala31 and cyclohexylalanine38 on that surface was as potent as astressin in binding to the CRF receptor and showed significant ACTH suppression when administered to rats.