The vasopressin precursor is composed of 3 domains in line, namely vasopressin, MSEL-neurophysin and a glycopeptide referred to as copeptin, which are separated during the processing. In guinea pig neurohypophysis, the precursor is partially processed so that a two-domain fragment, MSEL-neurophysin--copeptin, can be found along with free MSEL-neurophysin adn copeptin. Guinea pig copeptin has been sequenced. It is a glycopeptide composed of 38 amino acid residues rather than the 39 found in other mammalian copeptins. Compared with other copeptins, that from guinea pig shows a few substitutions and the deletion of one acidic residue, probably in position 32. This deletion might be responsible for incomplete cleavage by the trypsin-like processing enzyme.