1. Thermoneutral water immersion produces a physiological increase of thoracic blood volume, raises central venous pressure and increases urinary sodium excretion by a hitherto ill-understood mechanism. We have investigated whether this enhanced sodium excretion could be mediated by the recently discovered natriuretic factor, atrial natriuretic peptide (ANP).

2. During water immersion there was a highly significant (P < 0.001) twofold increase of the mean plasma ANP concentration and a doubling of the mean urinary sodium excretion. Both were unchanged during the control experiments.

3. These results are consistent with the hypotheses (a) that ANP is released into plasma in response to central blood volume expansion and (b) that it functions as a natriuretic hormone in normal man under physiological conditions.

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