The pituitary gland, brain, and various other tissues contain endorphins. These can be divided into at least two groups: peptides derived from beta-LPH (Group 1), and enkephalins and related peptides (Group 2). In addition, there seem to be still unidentified peptides. Recent studies have revealed heterogeneity in endorphin moieties, such as different chain lengths and the presence of acetylated forms. Such heterogeneous peptides may have biologic activities different from those studied with synthetic beta-endorphin or Met- and Leu-enkephalins. Further studies should clarify the exact nature of native endorphins present in various tissues and the actions of these native peptides. The interaction of endorphins with other putative neurotransmitters, as well as the nature and regulation of opioid receptors, must be studied to define all the features of the endorphin system.