Since the entire amino acid sequence of the adrenocort icotropin (adrenocort icotropic hormone, ACTH) from pig pituitaries was proposed (Bell, 1954; Shepherd et al., 1956b), a number of f ragments and analogues of A C T H have been synthesized and submitted to biological characterization. Concurrent with the remarkable advances in the field of peptide chemistry, the synthetic approach to the understanding of the s t ructure-act ivi ty relationships of polypeptide hormones has made a considerable progress during the last decade. Such studies are of great importance not only f rom a fundamental but also f rom a practical point of view, especially in connection with the problem of designing analogues which have some advantage over the natural hormones. A C T H is one of those peptides which have received the most extensive investigations along this line. In an a t tempt to review the recent investigations on the structure and function of A C T H in this article our attention is focused on the synthetic studies, which have enabled us to see the effect of the chain length and structural alteration of the hormone molecule on the ability to stimulate the adrenal steroidogenesis which is regarded as the major role of ACTH. Among some extra-adrenal activities exhibited by ACTH, only the melanocyte-st imulat ing activity will be mentioned briefly as a function of the structure of ACTH. For additional information the reader is referred to the excellent reviews and articles in which the subject is treated thoroughly or adequately (Hofmann, 1962; Hofmann and Yajima, 1962b; Lebovi tz and Engel, 1964; Li, 1962; Ramachandran and Li, 1967; Ramachandran, 1973; Schr6der and LiJbke, 1966; Schwyzer, 1963, 1964).