A method has been devised for collecting hypophysial portal blood from the anaesthetised guinea pig in order to measure the release in vivo of the neurohypophysial peptides, oxytocin (OT), vasopressin (AVP), neurophysin (NP), and the glycopeptide (GP) found at the carboxyl terminus of the AVP precursor. These peptides were measured in samples of portal and peripheral venous plasma by specific radioimmunoassays. The concentration of OT and AVP was 50- to 100-fold higher in hypophysial portal blood than in peripheral blood, with more OT than AVP usually present. There were correspondingly large amounts of NP and GP also present in portal blood. In particular, GP levels paralleled AVP levels over a wide range of concentrations and in virtually equimolar proportions. These results provide the first in vivo evidence which shows that, as for the magnocellular neurohypophysial system, GP is synthesised, processed and released in equal amounts with AVP from their common precursor in the subpopulation of parvocellular AVP neurons which project from the paraventricular nucleus to the median eminence.