WE HAVE observed that pituitary adrenocorticotrophic hormone (corticotrophin), added in vitro, stimulates the formation of corticoids by isolated rat adrenals and that the stimulation is approximately proportional to the amount of corticotrophin added (Saffran, Grad and Bayliss, 1952). This observation has been adapted to the bioassay of corticotrophin. METHOD Four adult Sprague-Dawley rats, of either sex, are used for each assay. The adrenals are prepared as previously described (Saffran, Grad and Bayliss, 1952). Care must be taken to keep the pair of adrenals from each rat together during the preparation. Four adrenals, one from each pair, are weighed together, bisected, and placed into a Warburg vessel containing 2 ml. of the Krebs-Ringer-phosphate-glucose medium. The remaining four adrenals are then weighed, bisected, and placed into a second Warburg vessel. The flasks are attached to Warburg manometers, flushed with oxygen, and are shaken at 38° for 1 hour. At the end of this period (the preincubation period) the media from both flasks are removed with suction and discarded.