The postnatal developmental course of the enhanced OT serum level of the vasopressin-deficient (homozygous) Brattleboro rat was investigated radioimmunochemically together with the response to treatment with Pitressin tannate. Compared with heterozygous Brattleboro (control) pups, in which serum OT appeared to have an adult value from birth onwards (about 10 pmol/l), homozygous rats had approximately 2-fold enhanced OT serum level throughout early development. Between day 55 and adulthood the levels of OT rose further to 40-50 pmol/l. A 3-day treatment with Pitressin tannate both in the period before or after the age (day 16) at which the polyuria of the homozygous Brattleboro mutant can be revealed, failed to reduce the serum OT. It was therefore concluded that the high OT serum levels in the vasopressin-deficient Brattleboro rat are not induced by osmotic imbalance, but probably originates from functional teratological aspects of the mutation.