The reviewer of this subject can approach his task in two ways; he can attempt to be exhaustive and include all the references to all the literature on the subject, or he can be selective and consider only that which has thrown new light in recent years. I propose to take the latter course. Since Braun Menendez wrote his review (1956), the subject has opened up by the discovery of the structure and amino acid sequence of hypertensin or angiotonin peptides [Peart 1955 (a), (b) and 1956; Elliott and Peart 1956 and 1957; Skeggs et al. 1954 (b), 1955 and 1956 (b)]. Synthesis of These and other pressor peptides soon followed, and subsequently other synthetic compounds have been prepared in which certain of the amino acids have been replaced by others not present in the natural compounds [Schwyzer et al. 1956, 1957 (a), (b), (c) and 1958 (a), (b), (c); Bumpus, SChwarz and Page 1957].