The following study was designed to evaluate plasma beta-endorphin (beta-EP) variations in healthy volunteers during thermoneutral head-out water immersion while prevalently in the standing position. The type of immersion was similar to that currently adopted for therapeutic rehabilitation. Plasma beta-EP was evaluated by RIA previous beta-lipotropin stripping and Sep-Pack cartridge methanol extraction. Plasma beta-EP levels significantly decreased during water immersion from a value of 12.71 +/- 2.04 pmol/l to 7.46 +/- 1.09 pmol/l at 15 min (P less than 0.05) and to 6.08 +/- 1.87 pmol/l at 30 min (P less than 0.01). Thirty min after the end of immersion, plasma beta-EP levels showed a slight increase to 6.98 +/- 1.88 pmol/l but were still significantly below the basal level (P less than 0.05). These results are consistent with the previously demonstrated decrease of ACTH and prolactin and the increase of plasma dopamine and decrease of norepinephrine, suggesting that thermoneutral head-out water immersion is not a stressful condition in healthy subjects. Further studies are necessary in order to clarify the mechanism involved in beta-EP decrease in normal subjects during thermoneutral head-out water immersion.