1. Nine patients with clinically unimportant heart disease or benign essential hypertension were given frusemide intravenously during right-heart catheterization.
2. Pressures in both atria decreased rapidly and in parallel. The magnitude of the pressure decrease was clearly related to decrease in plasma volume loss.
3. Plasma renin activity increased significantly after 5 min (P < 0·01), but did not correlate with plasma volume loss.
4. Venous tone in the forearm was unchanged.
5. It is concluded that the pressure reduction was secondary to plasma volume depletion through diuresis and that increased plasma renin activity was mainly caused by intrarenal changes.