Melanin-concentrating hormone (MCH) is a cyclic peptide (FIGURE 1) originally isolated from chum salmon pituitaries and characterized by Kawauchi et a/.'It is now well established that this heptadecapeptide is involved in regulating color change through stimulation of melanin granule aggregation within melanophores of melanocytes in teleost fi~ hes.~-~ Melanin dispersion in teleost melanophores is, indeed, under dual control by MCH and a-melanin stimulating hormone (a-MSH), an amidated peptide generated from proopiomelanocortin (POMC). S Paradoxally, high concentrations of salmon MCH (sMCH) cause pigment dispersion in amphibians and reptiles but also in fishes; this a-MSH-like activity may be attributable to sequences present outside the ring structure of s MCH.~.~ Apart from its action on the melanophore, sMCH may inhibit corticotropin-releasing factor (CRF) induced ACTH secretion and a-MSH release from fish pituitary cells in~ itro.*,~ More recent studies have shown that MCH release is enhanced under stressful conditions,''and that it may modulate the response of stress in fish." The discovery of sMCH and the availability of antiserum against it have made possible the characterization of the MCH counterpart in mammals. Using bioassay, radioimmunoassay, or immunohistochemical methods, an MCH-like peptide was seen throughout the brain but perikarya were found exclusively in hyp~ thalamus.'~-'~ The locations of MCH-like neurons were mapped in the ventral aspect of zona incerta (ZI) and in the lateral hypothalamus (LH) in rat'**'-'and human. I6 These areas are directly concerned with many important aspects of drinking and ingestive behavior^'^-^^ and are thought to contribute to the control of behavioral arousal and emotion. 21, 22 In view of these anatomical features and possible involvement of MCH in the control of hypothalamopituitary axis (HPA) in rat, 8x13 it was of interest to analyze in detail the MCH neuronal system in mammals. a This work was supported by La Fondation pour la Recherche MBdicale (" Credit excep-