Urinary antidiuretic hormone is very significantly correlated with sodium urinary output (r = 0,78; n = 54; p 0,001) observed during different sodium diet on eighteen healthy male subjects. Whereas antidiuretic hormone increase with sodium intake and renin-angiotensin-aldosterone system is suppressed, no prolactin variation is observed. Supine or upright position does not modify prolactin which decrease between 9 and 10 a.m. These results do not support a role for prolactin in sodium and water regulation, and show that sodium output has to be measured with urinary antidiuretic hormone.