The presence of vasopressin (VP) in pars distalis of rats and pigs was investigated. Using radioimmunoassay and bioassay of VP, a substance with immunological and biological properties of this hormone was found. This substance was not detected in the adenohypophysis of rats with diabetes insipidus. A partial purification of the VP-like peptide showed that it had the chromatographic and electrophoretic properties of VP. It could be identified with arginine vasopressin (AVP) in the case of the rat and lysine-vasopressin (LVP) in the case of the pig. In the Wistar strain, adrenalectomy induced progressively increasing concentrations of adenohypophysial VP. This increase was significant 15 days after surgery. It could be prevented by treatment with dexamethasone. These results indicate that the presence of VP in the anterior pituitary is related to the regulation of ACTH secretion.