1. The antidiuretic effect of hydrochlorothiazide in diabetes insipidus was investigated in rats with the hereditary hypothalamic form of the disease (Brattleboro rats).

2. Administration of hydrochlorothiazide in the food resulted in a marked fall in urine volume and a corresponding rise in osmolality. These effects persisted throughout the period of treatment (6–7 days).

3. Body weight and extracellular volume were significantly reduced in the thiazide-treated rats.

4. Hydrochlorothiazide caused an increase in urinary sodium excretion only on the first day of treatment. The resulting small negative sodium balance (in comparison with untreated rats) remained statistically significant for 2 days only. Thiazide-treated rats gradually developed a potassium deficit which was statistically significant from the fourth day of treatment.

5. Total exchangeable sodium, measured after 7 days of thiazide treatment, was not significantly different from that of untreated rats. However, plasma sodium was reduced in thiazide-treated animals, whereas erythrocyte sodium concentration was elevated.

6. It is concluded that the antidiuresis resulting from chronic hydrochlorothiazide administration is associated with a reduction in extracellular volume, but not with a significant overall sodium deficit. Hydrochlorothiazide appears to cause a redistribution of the body's sodium such that the amount of sodium in the extracellular fluid compartment is reduced.

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