Adrenocorticotropic hormone (ACTH) and corticosteroids exhibit an opposite effect on extinction of conditioned behavior in studies that were designed to elucidate the roles of the separate hormones: ACTH can inhibit extinction of conditioned avoidance behavior, while glucocorticosteroids—and some other steroids as well—can facilitate the rate of extinction. The study of the effects of ACTH on the behavior of intact animals received a new impetus after the discovery that fragments of this hormone containing the first 10 amino acids—or even smaller peptides such as ACTH-(C10)—could still exhibit a similar behavioral effect as ACTH. These peptides lack the well-known biological activities of the intact hormone such as stimulation of the adrenals. The behavioral effect of ACTH could be studied in intact animals independently of concomitant steroid action. Labeling these peptides as “ACTH-analogues” is more a matter of habit than of precise description because α- and β -melanocyte stimulating hormone (MSH), which exert a similar inhibiting effect on extinction of conditioned avoidance behavior as ACTH, also share the amino acid sequence that is known as ACTH.