1. Five normal subjects were studied before and during treatment with carbamazepine.
2. Plasma sodium, plasma and urine arginine-vasopressin and urine osmolality were measured during a day of water deprivation, before and during drug treatment.
3. During treatment with carbamazepine plasma sodium increased whereas plasma and urine arginine-vasopressin and urine osmolality decreased. Plasma and urine arginine-vasopressin were significantly correlated with urine osmolality. However, carbamazepine did not affect the osmolality of urine produced by the kidney, in response to endogenous arginine-vasopressin.
4. Plasma and urine arginine-vasopressin were significantly correlated with plasma sodium on both control and drug-treatment days, but the relationships of plasma and urine arginine-vasopressin to plasma sodium were different during carbamazepine treatment, as compared with the control period.
5. It is suggested that the threshold of the hypothalamic osmoreceptors for release of arginine-vasopressin is modified by carbamazepine, and that this may be either a direct action or secondary to another action of the drug.