Apparent discrepancies in published assignments of the PMR spectrum of Met5-enkephalin were due to the existence of 2 forms, cationic and zwitterionic. The PMR parameters of these 2 forms indicated different secondary conformations. The PMR parameters of the zwitterionic form were indicative of a 2-5 .beta.-I turn but no secondary conformation could be suggested for the cationic form with the present data.