1. The effect of progesterone on renal haemodynamics and intrarenal sodium handling was evaluated in thirteen normal men on a constant diet. Clearances were measured during maximal water diuresis and again 4–7 days later, this time 3 h after progesterone was given intramuscularly. Seven additional studies were performed 3 days after progesterone administration. Another four tests were performed on volunteers who had manifested renal ‘escape’ from the sodium-retaining effect of deoxycorticosterone acetate.

2. In acute progesterone studies glomerular filtration rate was unchanged, whereas effective renal plasma flow increased, so that filtration fraction decreased significantly. A similar increase in urinary sodium occurred whether subjects received a low or high sodium diet. Indices which related to the distal delivery of filtrate (fractional urine flow and the sum of fractional free water and sodium clearances) increased significantly in both groups. The progesterone-induced increase in sodium excretion was not related to changes in plasma renin activity, renin substrate or urinary aldosterone. After 3 days of progesterone, the increase of sodium excretion was less than in the acute studies and urinary aldosterone increased two- to four-fold. Progesterone failed to produce an acute increase in urinary sodium in subjects hyperexpanded by administration of exogenous mineralocorticoids.

3. Results suggest that the acute natriuretic action of progesterone is in part independent of aldosterone inhibition and that progesterone may inhibit sodium reabsorption at proximal as well as distal sites in the nephron.

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