Abstract. The concentration of β-endorphin-like immunoreactivity (β-ELIR) was determined in plasma and cerebrospinal fluid (CSF) of rats at various times following the implantation under Hypnorm® anesthesia of a permanent cannula into the cisterna magna. Plasma β-ELIR levels were highly increased immediately after the operation, and gradually returned to basal within 6 h. The β-ELIR concentration in CSF followed a completely different pattern. It appeared to be rather stable during the first hour after the operation, was decreased at 2 h, and showed a marked increase 24 h after surgery. Two days after the surgery, β-ELIR levels in CSF had decreased to a stable basal level which was maintained for at least 7 days. When at this stage animals were treated with Hypnorm, plasma β-ELIR was again highly increased 30 min and not 24 h after injection, but no changes were found in the β-ELIR concentration in CSF. These data indicate that implantation of a cannula into the cisterna magna induces postoperative changes in the CSF levels of β-endorphin that are detectable up to at least 24 h later. Moreover, the lack of correlation between CSF and plasma levels of β-ELIR points towards differences in the regulation of brain and pituitary pro-opiomelanocortin cells.