1. The effects of hypertonic saline and frusemide administered during hypertonic saline infusion were studied during antidiuresis and infusion of lysine-vasopressin in fifteen healthy subjects and seventeen patients with hypertension.
2. Rates of excretion of water and sodium were higher during salt loading in the hypertensive patients as compared to the healthy persons. On the other hand, frusemide administered during saline infusion had a greater effect in the normal subjects.
3. Free water reabsorption increased markedly in the first 10-min clearance period after frusemide injection in the healthy subjects, while in the hypertensive group there was no significant change. This suggested that the diuretic may have a more marked proximal tubular effect in the healthy persons and/or a more intensive Henle's loop action in the hypertensives. However, the latter possibility was not supported by data concerning the correlation between solute excretion and free water reabsorption.
4. It is suggested that sodium transport is competitively inhibited in the same segment(s) of the nephron by frusemide and hypertonic saline and that this may explain the decreased effect of frusemide in the hypertensives.