WE HAVE previously observed (Saffran and Serially, 1955a) that neurohypophysial tissue plus arterenol (norepinephrine) strongly stimulate the release of corticotropin (ACTH) from anterior pituitary tissue in vitro. A corticotropin-releasing factor (CRF) has been found in posterior pituitary extracts and has been shown to be distinct from vasopressin and oxytocin. METHOD The detection of CRF activity is described in detail by Saffran and Schally (1955a). It consists of incubating rat anterior pituitary tissue with the test substance in bicarbonate- buffered, oxygenated medium at 38° for 1 hour in the presence of 0.0004 M DLarterenol. HCl and 0.004 M ascorbate and then measuring the ACTH released into the medium, using the in vitro bioassajr of Saffran and Schally (1955b). The results are expressed in milliunits (mu.) of ACTH released by 1 mg. of anterior pituitary tissue in 1 hour, accompanied by the 95% confidence limits of the ACTH assay. In most experiments the anterior pituitary tissue from each animal was distributed equally between two flasks