Synthetic lysine vasopressin (LVP) was assayed in the in vivo corticotropin releasing factor (CRF) assay (nembutal-morphine blocked rat) of R. Guillemin. The only modification of the CRF assay was an increase from a 25-min. to a 35-min. interval between the nembutal and morphine injections. Release of corticotropin (ACTH) was determined by the increase of corticosterone (compound B) in 2 ml of plasma. Levels of compound B for saline injected controls were the same as for uninjected controls (8 ug/100 ml.) The sample of LVP used (JM V 68/16 of V. du Vigneaud) assayed 262 U/mg pressor activity in the dibenamine-treated rat (Dekanski assay). Between 8 and 67 mU of pressor activity, CRF activity vs log dose LVP showed an excellent linear relationship: at 16.8 mU LVP, compound B increase was 39%; at 33.5 mU, 81%; at 67 mU, 125%. In the 24 hr. hypophysectomized rat the plasma compound B controls were 2.4 + 0.1 tig/100 ml and injection of 67 mU LVP gave no increase over controls; this effect of LVP is therefore "CRF-like" and not "ACTH-like".