Orexins/hypocretins are recently discovered neuropeptides synthetized mainly by neurons located in the posterolateral hypothalamus. Hypocretin-1 and -2 are the same peptides as orexin-A and orexin-B. Orexin A is a 33 amino acid peptide with N-terminal pyroglutamyl residue and two intrachain disulphide bonds. Orexin B is a linear peptide of 28 amino acids. These two peptides are potent agonists at both the orexin-1 (OxR1) and orexin-2 (OxR2) receptors. Orexin-A is selective ligand for OxR1 and OX2 binds both orexins. The structure of orexins and their receptors is highly conservative in mammals. Orexin A sequence is identical in several mammalian species (human, mouse, rat, bovine and porcine). Intracerebroventricular administered orexin-A stimulates food intake and energy expenditure. Orexins are also involved in the regulation of neurohormones and pituitary hormones secretion as well as in the control of cardiovascular and sleep-wake function. Orexins also play a role in the pathogenesis of narcolepsy. Mutation in the gene coding preproorexin or OxR2 receptor gene results in narcolepsy in mice and canine. In patients with narcolepsy orexin neurotransmission was altered and orexin level in cerebrospinal fluid was undetectable.