The biochemical characterization of the enkephalins and the enkephalin-containing polypeptides and their biosynthesis has reached an advanced state. From the structural data showing great similarities between human and beef proenkephalin there is strong evidence for physiological roles for some of the extended enkephalins and for at least two of the ECPs. If, like the different hormones contained within proopiocortin, the different ECPs in proenkephalin perform different coordinate functions, their cosecretion with the catecholamines from the adrenal gland suggests that these functions play a role in the response to stress. It appears that proenkephalin represents another multi-hormone precursor, like proopiocortin, that is processed by proteolysis to yield a variety of peptides active in both the endocrine and nervous systems. The research effort must now turn toward resolving the nature of the physiologic functions these peptides perform.