1. In this review the structure-function relationships of the different vasopressin prohormone domains are dated and discussed, with special reference to the neurophysin and glycopeptide domains. 2. The primary structures of the currently known neurophysins and glycopeptide sequences are compared and discussed. 3. The hormone-binding and aggregational properties of neurophysin are reviewed and related to a possible function within the regulated secretory pathway. 4. It is proposed, based on the properties reviewed here as well as our own data shown here, that the sorting of the vasopressin prohormone is initiated by hormone binding, which triggers aggregation of the prohormone into the characteristic dense cores of the regulated secretory pathway. 5. This may suggest that prohormone sorting into the regulated secretory pathway is, in general, determined by noncovalent, intramolecular interactions that promote aggregation.