Department of Anatomy, King's College, The Strand, London, WC2R 2LS (Received 13 January 1975) It has been found that the incubated posterior pituitary of the mouse is capable of incorporating [3H]phenylalanine into corticotrophin (ACTH), and also that active ACTH synthesis occurred under conditions when no synthesis of melanocyte-stimulating hormone (MSH) was taking place, suggesting that synthesis of each hormone is regulated separately. Agouti mice (CBA/Ca strain) of both sexes were used. Posterior pituitaries were dissected free from anterior pituitary tissue and, in groups of 8–10, incubated at 37 °C in 1 ml bicarbonate-buffered Krebs–Ringer solution gassed with O2 : CO2 (95:5, v/v) and containing 1 mg bovine serum albumin and 20 μCi (2 nmol) l-[4-3H] phenylalanine. After 30 min, the radioactive medium was removed and replaced by 1 ml Krebs–Ringer solution containing albumin and 1 μmol l-phenylalanine. After 5 min, the glands were removed and homogenized together in 0·2 ml ice-cold