1. Carbamazepine had an antidiuretic effect in six patients with neurohypophyseal diabetes insipidus due to various causes. The effect was qualitatively similar to that of antidiuretic hormone.
2. At the time of maximal antidiuresis plasma arginine vasopressin levels, measured by radioimmunoassay, were below the limit of detectability in all patients.
3. In one of two control subjects a temporary decrease in water excretion was observed during administration or carbamazepine.
4. In the two control subjects plasma arginine vasopressin levels dropped from normal to below the level of detectability during administration of the drug.
5. It is concluded that the antidiuretic effect of carbamazepine is not the result of releasing endogenous antidiuretic hormone or of prolonging its half-life.
6. The most likely explanation for the observed results is either a direct effect of carbamazepine on the distal renal tubular cell or an increase of renal responsiveness to endogenous antidiuretic hormone.