Immunohistochemical analysis revealed the colocalization of ACTH-related immunoreactive forms and Î±-MSH-related immunoreactive forms in corticotropic cells of the anterior pituitary of larval Ambystoma tigrinum. Subsequent analysis of acid extracts of anterior pituitaries obtained from larval, neotene, and postmetamorphic adults indicated that the molar ratio of ACTH(lâ€ 39) to Î±-MSH was, respectively, 2.5:1, 1:1.5, and 1:1.3. However, in sexually mature adults the molar ratio of ACTH(lâ€ 39) to Î±-MSH was 8:1. These results indicate that before and immediately after metamorphosis, both ACTH(lâ€ 39) and Î±-MSH are major end products of corticotropic cells in this species. As postmetamorphic animals reach sexual maturity, ACTH(lâ€ 39) is a major end product of corticotropic cells and only trace amounts of Î±-MSH are produced. Thus in A. tigrinum the proteolytic processing of ACTH in corticotropic cells is developmentally regulated. The Î±-MSH related peptide produced by the corticotropic cells appears to be an ACTH(lâ€ 13)amide-like form.