The passage of the vasopressin analogue, 1-deamino-8-D-arginine-vasopressin (DDAVP) into the cerebrospinal fluid (CSF) following intravenous (i.v.) administration was studied in human subjects. After i.v. injection of DDAVP, resulting in plasma levels exceeding normal plasma concentrations of vasopressin (AVP) by 100-fold, serial blood and ventricular CSF specimens were sampled during 2 h. Plasma and CSF were analysed by radioimmunoassay using two different AVP antisera: one specific against natural AVP and another cross-reacting 100% with DDAVP. No changes in the joint concentrations of DDAVP + AVP in CSF were found following i.v. infusion of DDAVP when compared with the concentration before the infusion. Neither was any difference between CSF concentrations of AVP and DDAVP + AVP observed at any sampling time following the i.v. DDAVP administration. Hence, it is concluded that no measurable amounts of DDAVP can be found in the CSF for up to 2 h after i.v. administration of the peptide, and that for all practical purposes a blood-CSF barrier to DDAVP exists.