Melanin-concentrating hormone (MCH) is a cyclic peptide originally isolated from chum salmon pituitaries, then structurally characterized from rat hypothalami. In the fish, MCH induces melanin concentration within the melanophores and may inhibit ACTH secretion in vitro and under stressful conditions in vivo. In the rat, MCH modulates ACTH release in vivo and oxytocin secretion in vitro. However, on the basis of neuroanatomical features, that is, cell bodies almost exclusively confined to the lateral area of the hypothalamus and the zona incerta, while fibers are observed throughout whole rat or human brains, this peptide appears to participate as a neurotransmitter/neuromodulator in the control of goal-oriented behaviors and/or general arousal in mammals. The knowledge of structural and regulatory features of the MCH precursor, mRNA, and genes at the cellular and molecular levels has recently made great progress. (1) The cells expressing MCH and associated peptides have been defined conjointly using radioimmunoassay, immunocytochemistry, and in vitro and in vivo molecular hybridization techniques. (2) The organization of the precursor deduced from cDNA cloning has been established and led to the discovery of two novel putative peptides named NEI and NGE. (3) The regulation of MCH mRNA and peptide production has been explored during the course of development in rodent and human and under a variety of paradigms (neurogenic or osmotic stress, hormonal stimuli, etc.). (4) The structure of the MCH genes has been determined in salmon, rat, mouse, and human and revealed striking exon-intron organization differences between fish and mammals. Strong homology, with a likely functional implication, was found between salmon MCH mRNA and 7SL RNA, a structural RNA involved in protein translocation. Furthermore, a variant gene that may encode slightly different MCH was found exclusively in primates. (5) Chromosomal assignment of the authentic and variant MCH genes in rat and human indicates that these genes may be good candidates involved in neurodegenerative or psychiatric disorders. Based on the framework of these studies, a working model of MCH regulation/function in mammalian brain is finally proposed.