We examined the role of vasopressin (VP) in brain-adrenal function by comparing the plasma corticosterone concentrations of rats genetically deficient in VP [Brattleboro rats, homozygous for diabetes insipidus (Dl-homo)] with those of control animals. Rats were subjected to stressful situations and exogenous ACTH, and their adrenal sensitivity to ACTH was assessed in vitro. Some Dl-homo rats were treated chronically with VP and their plasma corticosterone concentrations were examined during stress. We confirmed that Dl-homo rats have lower than normal plasma corticosterone concentrations during stress and this was at least partially due to adrenal glands relatively insensitive to ACTH.