By simultaneously irradiating the resonances of 2 selected nuclei in a macromol. with selective radio-frequency fields, one can measure the cross-relaxation rate (Overhauser effect) between 2 sites without perturbations due to spin-diffusion effects involving other spins in the mol. Applications are shown to Ha to Tyr-23 and Ha of Cys-30 in basic pancreatic trypsin inhibitor (BPTI). [on SciFinder (R)]