Uptake of arginine-vasopressin, VP, at the luminal side of the blood-brain barrier (BBB) was studied by means of an in situ brain perfusion technique in the guinea-pig. Kinetic experiments revealed a saturable peptide influx into the parietal cortex, caudate nucleus and hippocampus with K m between 2.1 and 2.7 μM, and V max ranging from 4.9 to 5.6 pmol· min− 1· g− 1. The non-saturable component, K d, was not significantly different from zero. Influx of VP into the brain was not altered by the presence of the peptide fragments: VP-(1–8), pressinoic acid and [pGlu 4, Cyt 6] VP-(4–9) at 4.5 μM, nor yet by the aminopeptidase inhibitor, bestatin (0.5 mM) and the l-amino acid transport system substrates, l-tyrosine and l-phenylalanine at 5 mM. At a perfusate concentration of 4.5 μM, the V 1-vasopressinergic receptor antagonist, d (CH 2) 5 [Tyr (Me) 2] VP, reduced VP influx; regional K i values, assuming that the observed inhibitions were purely competitive, ranged between 4.7 and 8.5 μM. It is concluded that there is an apparent cerebrovascular permeability to circulating VP due to the presence of a carrier-mediated transport system for the peptide located at the luminal side. The mechanism for VP BBB uptake exhibits no affinity for peptide fragments and large neutral amino acids, but requires reception of the intact molecule, which may be the same initial step for both the BBB VP transporter and the V 1-receptor.