1. Vasopressin was administered to normal men in metabolic balance for periods of 5–10 days under conditions of water restriction or overhydration. Likewise, oxytocin was administered to two normal men for 10 days.
2. The effects of both neuropeptides on plasma renin activity, aldosterone excretion rate and sodium balance were observed.
3. In the absence of overhydration, vasopressin had no demonstrable effect upon plasma renin activity, aldosterone excretion rate or sodium balance. During overhydration body weight gain and plasma dilution were followed by natriuresis; the associated changes in plasma renin activity and aldosterone excretion, however, were unimpressive.
4. The prolonged administration of oxytocin for 10 days under conditions of normal hydration failed to influence sodium excretion, plasma renin activity or aldosterone excretion.
5. It is concluded that in normal man changes in circulating levels of vasopressin or oxytocin do not play a physiological role in the control of sodium excretion.