A B s T R A C T The present study was designed to examine the question of whether or not there is a natriuretic hormonal substance involved in the renal regulation of sodium balance.For this purpose, procedures for concentration and fractionation of plasma and urine samples and a sensitive bioassay for demonstrating changes in renal sodium excretion were developed. The natriuretic assay utilized rats with mild diabetes insipidus which were maintained in salt and water balance.Using these approaches a natriuretic humoral substance was demonstrated in plasma and urine from normal man and sheep, and in patients with primary aldosteronism or essential hypertension.It seems likely that this substance participates in day to day regulation of sodium balance because it was not detectable in sodium-depleted subjects and it consistently appeared in the sodium-loaded subjects.The hormonal agent may not act immediately and its activity can be apparent for up to 3 hr. Full expression of its activity requires that the assay animals be appropriately volume expanded. This suggests that the increases in sodium excretion mediated by this hormonal substance depend in part on the coparticipation of other physical and perhaps humoral factors.This natriuretic substance appears to be of large molecular weight or carried by a large molecule. The data suggest that it acts, at least in part, to block sodium reabsorption in a more distal portion of the tubule.