Procedures are being developed for use of 15N and 13C for structural analysis in solution by nuclear magnetic resonance spectroscopy. The procedures are nondestructive and applicable to mixtures of compounds. The major objective of development of equipment of greater detection sensitivity which permits routine NMR sttudies of 15N at its low natural-abundance level has been achieved and useful spectra have been obtained of alkaloids, peptides, enzymes, amino sugars, protamines, nucleotides, nucleosides and t-RNA. Special attention is being given to the 15N NMR spectra of histidine in biologically important systems for the purpose of elucidating the function of histidine in the enzymatic hydrolysis of peptides. The interplay between the tyrosine and zinc at the active site of the enzyme, carboxypeptidase A, is being studied by 15N NMR of an arsonophenylazo substituent on the particular tyrosine which is labeled in the azo group with 15N.