Pituitary adenylate cyclase-activating polypeptide (PA CAP) which belongs to the secretin/glucagon/VIP family has been originally isola ted from the sheep hypothalamus on the basis of its ability to stimulate cAMP fo rmation in culture rat anterior pituitary cells. Post-translational processing of the PACAP precursor generates two biologically active molecular forms, PACAP -38 and PACAP-27. The primary structure of PACAP has been remarkably conserved during evolution. The sequence of PACAP-27 exhibits substantial similarities w ith those of vasoactive intestinal polypeptide (VIP), glucagon and secretin. The gene encoding the PACAP precursor is widely expressed in brain and various peri pheral organs, notably in endocrine glands, gastro-intestinal，uro-genital tra cts and respiratory system. In vivo and in vitro studies have shown that PACAP exhibits multiple activities especially a trophic activity during ontogen esis, notably in the adrenal medulla and the central nervous system. The biologi cal effects of PACAP are mediated through three distinct receptor subtypes which exhibit differential affinities for PACAP and VIP. The PAC1 receptor, which sho ws high selectivity for PACAP, is coupled to several transduction systems. In co ntrast, VPAC1 and VPAC2, which bind with the same affinity for PACAP and VIP, ar e mainly coupled to the adenylyl cyclase pathway. In conclusion, PACAP is neurop eptide, and it functions as a hypothalamic hormone, neurohormone, neuromodulator , vasodilator, neurotransmitter or trophic factor in the brain and the various o rgans.