Longitudinal nuclear magnetic relaxation in coupled two-spin systems is discussed in terms of magnetization transfer modes, a generalization of the magnetization modes commonly used in the discussion of one-dimensional NMR relaxation expts. The symmetry properties of the transfer modes and their relationship to the elements to the usual Redfield relaxation matrix are discussed. Exptl. strategies for measuring the amplitudes of the transfer modes are discussed and various selective two-dimensional exchange (\"soft-NOESY\") methods are proposed. Exptl. data are presented for Exifone, a small org. mol., and the auto- and cross-correlation spectral densities, the magnitudes of the proton chem. shift anisotropies, and the strengths of the dipolar interaction are derived from the spectra. [on SciFinder (R)]