The influence of synthetic ACTH<sub>1-24</sub> and acth <sub>1-24</sub> on rat brain RNA was studied. ACTH<sub>1-24</sub> (5 IU/100 g s.c.) treatment resulted in a small decrease in labelling of brain stem RNA (-12%), measured 30 min after the injection of [5-<sup>3</sup>H] uridine (100 µCi; s.c), without affecting the distribution of radioactivity of the acid-soluble precursor pool. Furthermore, a small and transitional reduction of total brain stem RNA was found. Similar treatment of adrenalectomized rats (36 h after surgery) resulted in a marked increase (+ 40%) in labelling of brain stem RNA, without affecting the distribution of radioactivity in the acid-soluble pool. In both intact and adrenalectomized rats, treatment with ACTH<sub>1-24</sub> did not affect RNA of cortex cerebrum and cerebellum. ACTH<sub>1-10</sub>, a fragment of ACTH without corticotropic activity in vivo, exhibited no effect on brain stem RNA in either intact or adrenalectomized rats, nor did corticosterone (1 mg/100 g) affect adrenalectomized rats.