The chemical synthesis of two dimers of the sequence 11 to 24 of adrenocorticotropin is described. The two monomers were covalently linked through their N-or C-termini, respectively, using N-acetylglutamic acid or lysine amide as spacers. Compared to the monomer, the first dimer showed moderately, the second strongly increased inhibitory potency upon steroidogenesis in isolated adrenal cells.