The classical view of the relationships between the structures of adrenocorticotropin (ACTH) and the melanotropin-related peptides holds that the entire sequence of amino acids 1-24 is required for full corticotrophic activity. 1 ' Although activity is present in the heptapeptide 4-10 core, other sequences contained in the regions 1-4 and 11-24 are necessary for full binding to the ACTH receptor. The apparently weak activity of a-melanocytestimulating hormone (a-MSH) and related peptides was explained on the basis of a partial interaction with the ACTH receptor ,'·5 and it was assumed that these peptides, therefore, showed essentially the same type of activity as ACTH. The possibility that the glomerulosa (or reticularis) zones of the adrenal cortex may not respond in the same way as fasciculata cells, which are the major source of secreted glucocorticoids, was not examined.