The concentration of β-endorphin-immunoreactivity (βE-IR) in cerebrospinal fluid (CSF) and plasma of rats was determined following intracerebroventricular (ICV) treatment of conscious animals with substances known to stimulate the release of βE and other pro-opiomelanocortin (POMC)-derived peptides at the level of the anterior and intermediate lobes of the pituitary. The β-adrenoceptor agoinst isoproterenol (ISO) did not influence the concentration of βE-IR in CSF collected 5–60 min after ICV administration of doses ranging from 3 to 30,000 pg/rat. Plasma βE-IR levels, however, were significantly increased 20 min following ICV injection of 30,000 pg ISO. ICV treatment of animals with ovine corticotropin-releasing factor (CRF; 30–30,000 pg/rat) also did not affect CSFlevels of βE-IR, whereas CRF in a dose of 30 pg significantly decreased, and in doses of 300–30,000 pg enhanced plasma βE-IR concentrations as determined by 20 min following treatments. ICV injection of arginine8-vasopressin (AVP) in doses of 10–1,000 pg/rat dose-dependently elevated the βE-IR concentration in CSF without affecting plasma βE-IR levels. This AVP-induced increase in CSF βE-IR was maximal 20–35 min and βE-IR levels had returned to basal 60 min following treatment. The data indicate that AVP and not ISO and CRF is a stimulator of CSF levels of βE-IR. As βE-IR in CSF likely originates from brain POMC neurons, these results suggest the hot vasopressin may be a physiological regulator of brain POMC activity, and may act as a releasing factor for POMC-derive