The measurement of the migration of longitudinal 2-spin order by 2-dimensional (2D) exchange spectroscopy or by selective 1-dimensional (1D) methods (2D or 1D zz-exchange spectroscopy) allows unambiguous differentiation between chem. exchange and cross-relaxation processes in complex nuclear spin systems, provided a fragment with at least 2 coupled spins is transferred from 1 site to another in the exchange process. Compared to conventional 2D exchange spectroscopy, zz-exchange studies have the advantage that cross-peaks due to migration of zz-order usually suffer less from overlaps than the corresponding cross-peaks in 2D exchange spectra which often appear close to the diagonal. An attractive feature of zz-exchange spectroscopy is that it allows one to distinguish cross-relaxation processes (nuclear Overhauser effects) that occur within scalar-coupled spin systems (e.g., within individual amino acid residues in proteins) from processes where magnetization is transferred between different spin systems (e.g., between different amino acid residues in proteins). [on SciFinder (R)]