1. This study was designed to evaluate variations in plasma β-endorphin, methionine-enkephalin, adrenocorticotropic hormone and serum prolactin in healthy volunteers during head-out water immersion.

2. Water immersion induced an increase in methionine-enkephalin plasma levels, which was associated with a significant fall in mean arterial pressure and heart rate.

3. Conversely, a suppression of plasma β-endorphin, adrenocorticotropic hormone and serum prolactin was detected during water immersion.

4. We suggest that a dopaminergic inhibitory control mechanism may be involved in regulating circulating levels of β-endorphin, adrenocorticotropic hormone and prolactin in normal subjects undergoing extracellular fluid volume expansion produced by water immersion.

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