Beta-endorphin (B-EP) is a 31 amino acid polypeptide secreted by the pituitary, brain, and various other tissues. It has potent opiate-like activity, can produce analgesia, and may play a physiological role in adapting the immune system to stress by affecting lymphocyte and complement binding activity. To investigate the effect of B-EP on pain threshold, we analyzed cerebrospinal fluid (CSF) and plasma concentrations of B-EP in 12 intact and 12 hypophysectomized (HYP) rats. Plasma and CSF samples were obtained from both intact and HYP rats 24 hours postsurgery. Pain threshold values were then assessed using the tail-flick test. Results showed a decrease in the plasma concentration of B-EP (p < 0.001), but no significant difference in the level in CSF.