1. The effects of replacing a 1% NaCl drinking solution with Na+-free water for 2 days on body weight and fluid and electrolyte balances were studied in adrenalectomized and sham-operated rats.

2. Eight weeks after operation, after the animals had been drinking Na+-free water for 2 days, some adrenalectomized animals (about 75%; designated group 1) experienced body weight losses which were outside the 99% confidence limits for the sham-operated rats (– 9.2 to + 5.3 g) whereas the remainder (designated group 2) were indistinguishable from the controls.

3. The body weight loss in group 1 was associated with negative fluid, Na+ and K+ balances. In group 2 rats, fluid balance was maintained as well as in the sham-operated rats, but their handling of Na+ and K+ was different.

4. In a separate experiment, plasma aldosterone, corticosterone, catecholamine and solute concentrations were measured in adrenalectomized rats from groups 1 and 2 (selected on the basis of body weight loss whilst drinking Na+-free water) and sham-operated rats, after drinking Na+-free water for 2 days.

5. In group 1 animals, plasma aldosterone levels were unmeasurable and corticosterone was extremely low (less than 5% of controls). In group 2, aldosterone was measurable but low, and corticosterone was higher than in group 1 but lower than in sham-operated rats. In line with these findings, animals in group 1, but not those in group 2, were hyponatraemic and hyperkalemia These results are consistent with the activation of latent adrenocortical tissue in the group 2 animals.

6. We suggest that monitoring body weight changes resulting from drinking Na+-free water for 2 days provides a useful, non-invasive test for reliably distinguishing between those adrenalectomized animals with and those without adrenocortical insufficiency.

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