Proenkephalin peptide F is an enkephalin-containing peptide found predominantly within the adrenal medulla and is co-packaged with epinephrine within adrenal medullary chromaffin granules. Both epinephrine and peptide F can be secreted in response to similar stimuli. Because of this co-secretion, they may possess similar biological functions (e.g., modulating immune function). The response of epinephrine and peptide F are regulated by acute exercise making it a very powerful stimulus to study the acute effects of exercise on the circulatory response patterns of peptide F concentrations in the blood.