Abstract The present study reports on the effects of behaviorally active, N-terminal fragments of ACTH on the incorporation rate of leucine into proteins in slices taken from the brain stem of hypophysectomized rats. In parallel with in vivo studies, the presence of ACTH 1–24 or ACTH 1–10-7- l-Phe in the incubation medium stimulates the incorporation of [ 14C]leucine into protein, whereas ACTH 11–24 is ineffective. No changes are found with ACTH 1–10-7- d-Phe, the analogue which exerted opposite effects both in behavioral situations and on leucine incorporation in vivo. These data suggest a direct action in situ of ACTH on central nervous structures. The sequence 1–10, which lacks appreciable corticotropic activity, appears to be sufficient to exert this neurotropic action of ACTH.