Abstract: The invention provides ligands for melanocortin receptors. For example, the invention provides the melanocortin receptor peptide ligand Ac-Nlc-Gln-His-(p(I)-D-Phe)-Arg-(D-Trp)-Cly-NH2, (SEQ ID NO: 1), where the iodo group is unlabeled or radioactively labeled. The invention additionally provides methods of assaying for melanocortin receptors in a cell or tissue such as brain. The invention also relates to pharmaceutical compositions comprising a pharmaceutically acceptable carrier and a melanocortin receptor ligand and to methods of administering the pharmaceutical composition to a subject. The invention further provides tetrapeptide ligands for melanocortin receptors and methods of altering melanocortin receptor activity.