The advanced electron spin resonance (ESR) techniques of ENDOR, ELDOR, and Saturation-Recovery are to be applied to nitroxide radical spin-labels for the determination of (1) the spin-spin relaxation time (T2), (2) the spin-lattice relaxation time (Tle), (3) the nitrogen nuclear relaxation time (Tln), and (4) intramolecular motions. Various model systems are to be studied in a program of applied research with two main objectives: first, to explore the possibility that these parameters will be useful indicators when spin-labels are incorporated into biological systems as probes of structure and functions, and second, to measure these parameters so that they may be used in the interpretation of spin-label spectra, particularly spectra obtained in the very slow tumbling domain by means of the new technique of Saturation-Transfer-Spectroscopy.