We propose a novel approach to markedly reduce rf power for both H-1 and observed nuclei during spin exchange for separated local field experiments. The rf power to satisfy the Hartmann-Hahn matching conditions during spin exchange for observed nuclei was arbitrarily reduced by alternating the directions of effective fields for H-1 nuclei with unequal duration times and amplitudes. The proposed techniques were compared experimentally with those developed previously by the authors. The rf power for observed nuclei and average H were reduced by factors of 9 and 2, respectively, for C-13 NMR signals of liquid crystalline 5CB. (c) 2005 Elsevier B.V. All rights reserved.