ACTH and ACTH receptor-like molecules were found at the examined stages of development (2, 4, 8, 12, 18, and 24 days post-hatching) in yolk sac, pronephros tubules, interrenal tissue, thymus, liver, spleen, cardinal veins, and skin of the teleost fish Dicentrarchus labrax. ACTH and the related receptor-like molecules show a similar distribution. LPS treatment at two different stages (8 and 24 days post-hatching) provoked both a release and an induction of ACTH-like molecules, suggesting an important role of this peptide to control the modifications in body homeostasis during the first period of the sea bass´ life, i.e., 30 days post-hatching, before the lymphoid cells have reached complete maturation.