1. The effects of intravenous chlorthiazide and frusemide on urinary osmolality were compared in 19 hyponatraemic oedamatous patients.

2. Frusemide (1 mg/kg) caused production of a dilute urine (urine/plasma osmolality ratio, Uosm./Posm., 1.64–0.84, P < 0.01) whereas chlorthiazide (10 mg/kg) did not (Uosm./Posm. 1.54–1.34, not significant).

3. The osmolar clearance (Cosm.) was higher after frusemide than after chlorthiazide (11.45 vs 4.99 ml/min, P < 0.01). When the doses of frusemide (0.25–0.5 mg/kg) and chlorthiazide (20 mg/kg) were chosen to give a similar Cosm. (7.25 vs 7.48 ml/min, not significant), the Uosm./Posm. was still lower after frusemide (2.20–1.00, P < 0.001) than after chlorthiazide (1.75–1.26, P < 0.01).

4. Exogenous vasopressin did not increase the low Uosm./Posm. after frusemide (1.00–1.00, not significant) but increased the ratio after chlorthiazide (1.34–1.68, P < 0.001).

5. These results indicate that frusemide, but not chlorthiazide, leads to the excretion of a dilute urine in hyponatraemic oedematous patients. This dilution is not due to a greater solution excretion but is associated with a resistance to the action of vasopressin.

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