1. Urinary and plasma levels of adenosine 3′:5′-cyclic monophosphate (cyclic AMP) after an intravenous injection of bovine parathyroid hormone (PTH) were measured in 12 patients on long-term lithium treatment and in nine control subjects. The maximum urine osmolality (Umax.) after an intravenous injection of desamino-d-arginine vasopressin (DDAVP) was also measured.

2. in all the control subjects and six of the patients, the Umax. after DDAVP exceeded 700 mosmol/kg. The cyclic AMP responses in these two groups did not differ significantly.

3. in the remaining six patients whose Umax. did not reach 700 mosmol/kg, the cyclic AMP response to PTH was significantly less than that of the controls.

4. A strong correlation was demonstrated in the patients between the urinary cyclic AMP response after PTH and the maximum osmolality after the administration of DDAVP.

5. These observations are consistent with the hypothesis that reduced adenylate cyclase activity contributes to the development of nephrogenic diabetes insipidus in patients on long-term lithium treatment.

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