In pregnancy there is a rise in glomerular filtration rate (GFR), plasma aldosterone levels, uric acid clearance and urinary kallikrein excretion. In toxemia all the above parameters tend to decrease. Progesterone has a diuretic effect which is usually related to aldosterone antagonism. We administered progesterone to normal women and observed that GFR, uric acid clearance and kallikrein excretion increased significantly, GFR from 103.0 +/- 13.7 ml/min to 118.0 +/- 18.0 ml/min (P less than 0.01), uric acid clearance from 9.0 +/- 3.6 ml/min to 14.3 +/- 4.0 ml/min (P less than 0.01), and urinary kallikrein excretion from 165 +/- 156 mU to 432 +/- 220 mU (P less than 0.01). Natriuresis and potassium excretion also increased from 15.9 +/- 6.4 mEq to 33.4 +/- 10.9 mEq (P less than 0.01) and from 7.6 +/- 2.7 mEq to 14.0 +/- 5.4 mEq, (P less than 0.01), respectively, suggesting that in this situation aldosterone antagonism is not relevant to explain the diuretic effects of progesterone.