Overhauser effects and hence internuclear distances can be measured accurately with selective expts. designed to suppress spin diffusion. It is essential to consider effects of both transverse and longitudinal relaxation during selective radiofrequency pulses. Fitting procedures allow one to refine selected cross-relaxation rate consts., and hence det. internuclear distances with improved accuracy. For the sake of illustration, selected cross-relaxation rates are detd. that correspond to short- and long-range distances involving two diastereotopic sugar protons in a double-stranded B-DNA dodecamer. Such distances are difficult to distinguish by traditional Overhauser methods because of spin-diffusion effects. [on SciFinder (R)]